I will no longer be updating this thread as new information becomes available. All of this information, with current updates, is available on my site blowback9.com under "Everything about the AR9". There you can browse or easily jump to individual sections for convenience.

This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted.

The first 9mm AR that I assembled had many issues I had to work through, requiring many hours of research and experimentation. This is information I found while researching all the problems. I know I would have appreciated having all of this organized and in one place, so here it is.

AR-pattern 9mm simple blowback basics:

- The original AR-pattern 9mm simple blowback operated SMG/carbines were developed by Colt. Owners report they use a heavy 16.0 oz. bolt, a carbine length (3.25") 5.5 oz. sliding-weight buffer (bolt+buffer recriprocating weight of 21.5 oz.), an aluminum buffer spacer (0.75") installed in the back end of the recoil spring, a standard carbine spring, and carbine buffer tube.
- The Colt-style magazine is based on the Uzi mag, with the mag catch moved and last round bolt hold open (LRBHO) capability added.
- Solid steel buffers were used at first, but they switched to sliding weight buffers after reports of out-of-battery discharges from bolt bounce.
- The 9mm bolt is about 0.75" shorter than the 5.56mm bolt and carrier. A buffer spacer limits the rearward travel of the shorter bolt and helps prevent bolt catch damage. It also helps prevent fired cases from getting jammed in the fire control group pocket of the lower. Extended length buffers (about 4.0” long) are available that are designed to eliminate the need for a buffer spacer.
- 22 oz. – 24 oz. is a bolt + buffer weight range that seems to work well with the vast majority of configurations and ammunition types.
- Bolts available on the consumer market generally seem to weigh 14 oz. - 15 oz.  Bolts weighing 16oz. are not common.
- 9mm buffers generally weigh from 5.5 oz. to around 12 oz.
- Aluminum cased ammo is generally not recommended in simple blowback systems.
- Simple blowback systems get dirty quickly and may be rough on parts. Keeping everything clean, lubricated, and carefully inspected should help prevent a number of potential problems.
- The most common problems that come up with 9mm blowbacks are ejection issues and chambering/feeding issues (see below for more info.)

What is “simple blowback” (a.k.a. direct blowback, plain blowback)?

- In a simple blowback firearm operating system, there are no locking bolt lugs, or any other mechanical locking or delay mechanisms, to keep the breech closed during firing. When a cartridge is fired, high pressure gasses push the bullet in one direction down the barrel, and the (now empty) cartridge case is pushed in the opposite direction out of the chamber. Movement of the thin-walled cartridge case has to be minimized until the bullet has left the other end of the barrel and the gas pressure can release. Otherwise, the thin walls of the cartridge case could burst from pressure as it is pushed out of the chamber, blowing cartridge case fragments and high pressure gasses in all directions and potentially causing injury.
- Slowing the extraction of the cartridge case is achieved by the heavy buffer and bolt behind the cartridge. The bolt and buffer want to remain at rest, resist the rearward push, and move slower than the relatively lightweight bullet heading in the opposite direction. In a properly weighted and balanced system, the bullet and the high pressure gasses fly out of the muzzle end of the barrel well before the fired cartridge case moves far enough to rupture.

Build vs. buy

- Many people are attracted to the idea of building their own 9mm simple blowback AR because of the cost savings over purchasing a completed firearm from a large manufacturer. Some people just want to build a completely custom 9mm AR or enjoy the challenge. Many people have successfully completed builds without any (or just minor) issues.
- Some people have had serious issues with their builds which resulted in the expenditure of a lot more time and money than they had originally planned. This may include replacing practically all of the parts until a combination is found that works properly; spending a lot of money on tools, ammo, and range trips; and spending months getting it all sorted out. It is possible to spend as much or more than the cost of a top quality complete 9mm AR.
- Purchasing a complete 9mm AR from a manufacturer with a good reputation, good customer service, and a solid warranty may have significant advantages. They should have already gone through all the troubleshooting, testing, ammo, money, and frustration to make sure their particular combination of parts works correctly. If it does have problems, it can usually be sent back for warranty repair and it will be their problem to figure out what is wrong and to make it work.

Parts choices make a difference

The following parts individually seem to solve various problems, or appear to cause fewer problems, with 9mm AR platform builds.

- A barrel with a generous feed cone.
- A ramped, proper weight, 9mm bolt from one of the larger reputable manufacturers.
- A carbine-length buffer tube with either: a 9mm-specific, carbine length, sliding-weight buffer, paired with a commercial buffer spacer; -OR- a 9mm-specific, extended-length, sliding-weight buffer.
- A total recriprocating mass (bolt+buffer weight) of 21.5 oz. to 23.5 oz.
- A standard strength recoil spring.
- A standard “mil-spec”-type semi-auto trigger set with "DPMS-style hammer" (most common hammer currently made).
- A standard strength hammer spring and trigger spring.
- A properly machined, in-spec., aluminum receiver.

Parts to avoid

Be aware that just because a part is advertised as "for 9mm" doesn't mean it's a good idea to use it. The following are a few of the parts that generally cause more problems in a 9mm blowback system and should either be avoided or never used at all.

- "9mm specific" hammer (notch cut in middle of face).
    Obsolete, and was never actually needed for 9mm. May cause bolt to jam in receiver.
- "AR-15 specific" hammer (notch, or "step", cut at front top of face).
    Obsolete, may cause bolt to jam in receiver.
- An unramped bolt.
    Causes excessive hammer pin stress/breakage.
- A light hammer spring.
    May cause doubling or runaway.
- A .308 rifle recoil spring.
    Completely unnecessary and may cause bullet setback/overpressure, trigger reset failures, nosedives, stovepipe jams, and conceal feed issues.
- A polymer receiver.
    Most are not designed for the violent blowback forces. May crack/break. Exception: KE Arms poly lower is reported to work with 9mm PCC builds with 5.4 oz. buffer minimum.
- A solid buffer (no sliding weight inside).
    Causes bolt bounce. May result in out-of-battery case ruptures during controlled pairs or rapid fire.
- A carbine-length buffer.
    Avoid, unless paired with a buffer spacer. Causes bolt rearward overtravel. May cause broken bolt catches, allows cartridge cases to fall into and jam the trigger group.
- An extra-power recoil spring
    Avoid, unless a higher bolt cyclic rate is specifically required to resolve hammer-follow issues.
- Aftermarket triggers designed for 5.56mm AR-15.
    Avoid, unless specifically approved by the maker for PCC, or has a proven history in PCC. There is no agreed-upon standard for 9mm bolt machining like 5.56mm bolts. Some triggers may work, some won't.



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This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted.

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Magwell adapters vs. dedicated PCC lowers

- Magwell adapters are an option for people with MG or SBR AR lowers, or anyone who would like to use their current AR lower for 9mm. Adapters may require some fitting and adjustment to work properly. Most include last round bolt hold open (LRBHO) capability. Some adapters cost as much or more than dedicated lowers.
- Magwell adapters are available that use mags for Colt (modified Uzi), Glock, S&W M&P, Beretta 92, Sig P320, and others.
- Some magwell adapters use a separate mag catch/release button, usually located at the bottom of the magwell adapter. The original mag catch in the lower holds the adapter in place. To prevent ejecting the magwell adapter by accident, some users install a CA/NY compliance "bullet button" or internal magazine locking system in the lower receiver.
- Dedicated lowers often come from the factory with the mag catch and ejector installed. Some have LRBHO capability, some do not.
- There are lowers available for Colt, Glock, MP5, Scorpion Evo, and other mags.

Extra-power carbine or .308 rifle recoil springs vs. standard carbine recoil spring

- An extra-power or .308 recoil spring should never be used as a substitute for having a proper weight buffer and bolt.
- Colt’s 9mm carbines are reported to use a standard carbine recoil spring.
- Some people have reported better operation with their setup with an extra-power recoil spring, while others have found it provided no real benefits or caused problems.
- An extra-power spring may slow the rearward acceleration of the bolt slightly.
- An extra-power recoil spring forces the bolt forward faster and harder than a regular spring when returning the bolt to battery. This reduces bolt cycle time, increases the force available to strip cartridges out of the magazine, and puts extra stress and wear on bolt catch parts, possibly causing breakage.
- Increasing the forward bolt speed may not give brass enough time to eject, trapping brass in the ejection port (stovepipe jam).
- Some users have reported that .308 springs forced the rounds to feed too quickly, causing the cartridge to ram into the bottom of the breech face (nosedives).
- .308 rifle recoil springs may force a cartridge to chamber after slamming the cartridge into the breechface, forcing the bullet backward and deeper into the case ("bullet setback"). This reduces the space in the cartridge case for the burning powder to expand, and can result in an overpressure situation when the cartridge is fired, potentially causing damage and injury.
- Bolt dwell time may be too short for reliable trigger reset when using a .308 recoil spring.

Anti-rotation pins or stainless steel pins

- It was found early on that 9mm simple blowback AR platform SMGs with 9mm-specific hammers and un-ramped bolts broke trigger group pins more frequently than the 5.56mm variant AR. Replacing the standard pins with stronger stainless steel pins increased the time before breakage would occur.
- Some people add anti-rotation pins to their AR to prevent the receiver pin holes from being damaged, which may be more important to someone with a registered SBR or MG lower.

Barrel length

- 9mm tends to increase in velocity as barrel length increases, with the largest gains generally seen as length increases up to 8".
- Beyond 8" of barrel travel, the average % velocity increase per inch is significantly less, and often inconsistent, depending on cartridge load.
- See Ballistics by the inch - 9mm results for details.
- Longer barrel lengths may cause some subsonic loads to become supersonic, affecting suppressor performance.
- Longer barrel lengths may cause poor terminal performance from hollow point ammunition designed for typical pistol barrel lengths.
- Shorter barrel lengths may result in the appearance of yellow flakes of partially burnt powder in the action, depending on the ammunition used.

Handguard length

- 9mm barrel length is measured by MOST manufacturers as the actual barrel length, chamber face to muzzle, since there is no barrel extension like with 5.56.
- The barrel sits about 9/16″ inside the receiver from the rear of the handguard.

 Generally:

- Handguard length = barrel length: Muzzle is 9/16″ inside the handguard.
- Handguard is 1/2″ shorter than barrel: Barrel covered by handguard, Muzzle will be about 1/16″ inside.
- Handguard is 1″ shorter than barrel: Muzzle outside handguard, threads partially exposed (7/16″ total), but thread shoulder is under the handguard.
- Handguard is 1 1/2″ shorter than barrel: Muzzle, threads, shoulder, and a little extra barrel exposed (15/16″ total).

The Law Tactical folding mechanism

- 9mm bolts are patterned after the 5.56mm bolt carrier, but they need to weigh significantly more to allow for safe operation of the simple blowback system. Additional weight is usually added in one of two ways: an additional weight plug is fastened inside the back of the hollow bolt, or the bolt is split into two parts with a removable solid heavy cylinder at the rear end.
- To use a Law folder, the bolt needs to be the type with an internal weight plug fastened inside the hollow rear of a full-length bolt.
- The inner diameter of the hole in the hollow back end of the bolt needs to be around .615", and the internal weight needs to be removable (they are usually held with a horizontal roll pin). This is because the Law folder uses a bolt extension that plugs into the back of a standard length hollow bolt carrier.
- The Law bolt extension weighs about 2.0 oz., so it will only make up some of the weight lost (usually 3.5 oz.) from removing the internal bolt weight. KAK makes a custom heavy extension plug (3.9 oz.) that is a drop-in replacement for PCC applications. Some people have successfully trimmed and reinstalled the internal bolt weight so that both the shortened weight and the Law extension fit properly. Others have used a heavier buffer to make up the lost weight.

Franklin Armory BFSIII in a 9mm AR

- Getting the Franklin Armory BFSIII to work properly in a 9mm AR can require some tuning. “Hammer follow” is the most frequently reported issue.
- “Hammer follow” appears to be the result of either “outrunning the trigger” (hammer releases as the bolt is still moving forward and comes to rest behind the bolt without firing a cartridge), or “bolt bounce” (hammer falls forward as the bolt “bounces” backwards, preventing the hammer from striking the firing pin and causing the hammer to come to rest behind the bolt). In both situations the hammer may be resting against the rear of the firing pin with an unfired cartridge in the chamber. A light primer strike may be evident.
- Buffers with sliding internal weights should help prevent “bolt bounce”.
- Getting the bolt into battery faster should help prevent “outrunning the trigger”. This may be possible by reducing overall bolt travel or increasing recoil spring pressure. The Franklin "medium" recoil spring (silver w/black end) that comes with the kit is reported to help..
- Success has been reported with the standard strength hammer and trigger springs, a standard carbine recoil spring, and a 9mm-specific buffer with sliding internal weights (either extended length, or carbine length in combination with a buffer spacer to limit bolt over-travel) that gives a total recriprocating mass (bolt+buffer weight) of about 21.5 oz. This may be a good baseline starting point for tuning.
- limiting use to quick binary "double taps" (pull-release-pause, pull-release-pause...) seems to prevent hammer follow. "Racing" the trigger through multiple binary cycles is more likely to cause hammer follow.
- All safety instructions and testing protocols provided by the manufacturer must be followed.

Putting a 9mm upper on a 5.56mm AR

- Some people have installed a 9mm magwell adapter of some type into a 5.56mm AR, swapped the 5.56mm upper for a 9mm upper, and started using it without any immediately apparent negative consequences.
- The typical 5.56mm buffer is much lighter than a 9mm-specific buffer. A light buffer may result in increased wear and stress on internal parts and could be dangerous to operate.
- Swapping the 5.56mm buffer for a 9mm-specific extended-length (about 4.0”) buffer may help reduce perceived recoil, reduce stress and wear on the system, limit bolt over-travel, reduce the chances of damaging the bolt catch during last round bolt hold open, and help prevent an empty case from falling into the trigger group pocket of the lower.

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(Edited for clarity/new information.)

This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted.

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Buffer weight

22 oz. - 24 oz. bolt+buffer weight (reciprocating mass) appears to work well with most 9mm blowback configurations and factory ammunition types. For most builds, this means using a buffer around 6.5 oz. - 8.5 oz.

[My personal setup has a short 4.5" barrel, a very heavy 27 oz. of total (bolt+buffer) weight, and a standard carbine buffer spring.  It functions with commercial bulk cheap 115gr., hot 124gr. NATO, and subsonic 147gr. ammo without any issues, suppressed and unsuppressed. Perceived recoil is noticeably softer with the heavier buffer/bolt compared to lighter weight buffer/bolt configurations I have tried.]

Lighter buffers may: increase wear/damage on parts, increase perceived (felt) recoil, and decrease cycle time.  Heavier buffers may: decrease wear/damage on parts, reduce perceived (felt) recoil, and increase cycle time.

These are just a few examples of various buffers (all weights approximate). Some are solid, some have sliding internal weights that help prevent bolt bounce, and some use elaborate designs and materials to reduce perceived recoil. Some are standard carbine buffer length (~3.25") and need a buffer spacer (see below), while others are extended length (~4") and should not need a buffer spacer.  

Standard carbine buffer, buffer spacer, extended length buffer:

[Image of buffers and buffer spacer]

Not generally recommended for 9mm, for comparison only:

3.0 oz. - 5.56 standard carbine
3.7 oz. - 5.56 H1 carbine
4.6 oz. - 5.56 H2 carbine
5.2 oz. - 5.56 rifle-length buffer

May work for a 9mm simple blowback setup:

5.5 oz. - 9mm carbine (solid, carbine - needs spacer)
5.5 oz. - Bravo Company H3 carbine (sliding weight, carbine - needs spacer)
5.5 oz. - Blitzkrieg elastomeric
6.0 oz. - Blitzkrieg hydraulic
6.0 oz. - B&T Hydraulic
6.1 oz. - Vltor A5H3 (sliding weight, extended)

These usually provide a 21.5 oz - 23.5 oz total bolt+buffer weight, depending on bolt weight:

6.5 oz. - Foxtrot Mike FM-9 (solid, WARNING: proprietary length! ~3.75" - needs custom spacer)
6.5 oz. - Slash’s HSS Tungsten (sliding weight, carbine - needs spacer)
6.8 oz. - Vltor A5H4 (sliding weight, extended)
7.4 oz. - Odin Works adjustable 9mm (sliding weight, carbine - needs spacer)
7.5 oz. - Macon Armory (sliding weight, extended)
7.5 oz. - Kaw Valley Precision (solid, extended)
7.7 oz. - Spikes tactical st-9x (solid, extended)
8.0 oz. - Vltor A5-SH2 (sliding weight, extended)
8.0 oz. - KAK Industries (solid, extended)
8.4 oz. - KAK Industries (sliding weight, extended)
8.5 oz. - Slash’s 9mm-Q Buffer (solid, extended)

"Extra heavy" - Often preferred by suppressor users, but still work well with most ammo loads:

10.0 oz. - KAK Industries (sliding weight, extended) **
10.0 oz. - Kaw valley Precision (sliding weight, extended)
11.0 oz. - MidwayUSA AR Stoner PCC (sliding weight, extended)
11.0 oz. - Stern Defense (sliding weight, extended)
12.0 oz. - Slash’s 9Q-T (sliding weight, extended)

** I have the KAK 10 oz. It weighs in at 10.8 oz. on my scale. It can be adjusted by swapping the internal sliding tungsten weights with steel weights ($1 each at KAK). All tungsten: 10.8 oz.; one steel: 10.0 oz.; two steel: 9.2oz.; three steel: 8.4 oz.; four steel: 7.6 oz.

Commercial buffer spacers

- The 9mm bolt is about 0.75" shorter than the 5.56mm bolt and carrier. A buffer spacer at the back of the recoil spring limits the rearward travel of the shorter 9mm bolt when using a standard-length (3.25") carbine buffer in a carbine buffer tube, and helps prevent bolt catch damage. It also prevents fired cases from getting jammed in the fire control pocket of the receiver.
- An extended-length buffer (~4") generally eliminates the need for a buffer spacer.
- Commercially-made buffer spacers (~0.75") are made by Spikes Tactical, New Frontier Armory, Ergo, Wolfpack Armory, and others.
- Buffer spacers are inserted into the opposite end of the recoil spring from the buffer, and inserted first into the buffer tube when reinstalling the spring.

U.S. quarters as a buffer spacer?

- It’s fairly common to find the suggestion to drop several U.S. quarters into the empty buffer tube behind the recoil spring to act as a cheap and customizable-length buffer spacer when using a carbine length (3.25”) 9mm buffer.
- A stack of U.S. quarters is usually cheaper than purchasing most commercial buffer spacers or extended buffers.
- They allow a user to customize the distance between the bolt catch and the bolt face at the fully rearward bolt position.
- If the space between the bolt and the bolt catch is reduced too much by the over-application of quarters, the bolt may override the catch before it has time to pop up into position, defeating the last round bolt hold open feature.
- Since quarters are nearly the same diameter as the inside of the buffer tube, the recoil spring sits on top of them, compressing the spring and causing it to apply more force when cycling.
- U.S. quarters may deform over time from the repeated impact of the buffer.

9mm hammers and “ramped” bolts

Typical AR hammer. Also called "DPMS style". Works with almost every 9mm bolt.

[Image of DPMS-style hammer]

- Avoid "9mm-specific" hammers with a notch in the middle of the hammer face. These hammers are still made, but are obsolete and frequently cause jams or other issues. 9mm-specific hammers are not beneficial or required for a 9mm AR.
- The original 9mm bolts were "unramped", meaning they were machined with almost no bevel under the rear of the firing pin where it resets the hammer. They were paired with a “9mm-specific” hammer with a large notch in the middle of the face that was designed to jam against the unramped area if the disconnector failed to catch the hammer (by accident, or in an attempt to make it "full auto").
- Unramped bolts were tested by users and found to work with most hammers. 9mm-specific notched hammers were unnecessary.
- Unramped 9mm bolts reset the hammer with a lot of force and put a lot of stress on the hammer pin.
- "Ramping” the 9mm bolt cuts a larger/deeper bevel (ramp) under the rear of the firing pin, giving the bolt more leverage for resetting the hammer, increasing the hammer reset time, reducing stress on the hammer pin, and rendering the "9mm specific" notched hammer design completely obsolete.
- Most current production bolts come “ramped” from the manufacturer. Un-ramped bolts can be made “ramped” by a number of machine shops.
- Avoid hammers with a 90 deg. notch cut in the very top front of the hammer face.  These so-called "AR-15 specific" hammers were designed to catch the firing pin collar in early semi-auto AR-15 "unshrouded firing pin" bolt carriers in case of disconnector failure. These hammers are obsolete and can jam the carrier in the receiver.
- More specific info here: LINK, under “Which hammer do I have, and will it work in my 9mm upper?”

Aluminum cased ammo

- Aluminum cased ammo appears to be more likely to cause problems in simple blowback operated firearms and the general consensus is to avoid it.
- Some people have used it successfully, some have had poor results.
- Torn rims, torn cases, ruptured cases, and extraction failures may be more likely in a simple blowback 9mm AR when using aluminum cased ammo.
- Some people have reported it worked OK with an 8 oz.+ buffer, but had case failures/ruptures with lighter buffers.

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[Post edited to fix broken link and clarification]

This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted.

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Bulged or ruptured fired cartridge cases

- This may occur when aluminum cased ammo is used in a simple blowback operated system. See “Aluminum cased ammo” for more.
- If the cartridge doesn’t seat fully in the chamber, the unsupported portion may bulge or rupture when fired.
- Cartridge cases that have been resized/reloaded several times may have stretched, leaving a portion of the rear of the case unsupported by the chamber.  Check case length in a case gauge.
- The cartridges may not have proper dimensions or powder loads. This tends to be a more frequent problem with reloaded ammunition.
- There may be a buildup of residue or a blockage in the chamber which prevents a cartridge from seating fully.
- The breech may be opening too soon in the firing cycle. An AR platform 9mm simple blowback system should use a 9mm-specific buffer, along with a proper weight 9mm bolt. 5.5 oz. is generally considered to be the minimum buffer weight that should be used for a 9mm simple blowback AR.
- Bolt bounce may be a problem. See “Bolt bounce” for more.

Bolt rearward over-travel

- Broken bolt catch
- Bent or damaged bolt catch
- Fired cartridge case jammed in the receiver trigger group recess

- The typical 9mm bolt is about 0.75" shorter than the typical 5.56mm bolt/carrier. This is because the 9mm bolt face is essentially machined into the front of the 5.56mm carrier and so doesn't have a protruding bolt like 5.56mm. As a result, when the 9mm bolt stops moving rearward, the bolt face is about 0.75" further back in the action, exposing the front of the trigger group recess in the lower receiver. Fired cartridge cases can fall down into the trigger group recess, causing a jam that requires disassembly to resolve.
- Because the bolt face sits further back in the action at the end of travel, on the way forward again the heavy 9mm bolt has extra runway to gain momentum and can slam hard into the bolt catch, possibly damaging or breaking the bolt catch assembly.
- A commercially available buffer spacer fits in the rear of the recoil spring and limits the rearward travel of the bolt when using a carbine length buffer.
- Extended-length buffers are designed to limit the travel of the bolt without the need for a buffer spacer.
- It’s fairly common to find the suggestion to drop several U.S. quarters into the empty buffer tube behind the recoil spring to act as a cheap and customizable-length buffer spacer. See “U.S. quarters as a buffer spacer” for more.

Example of bolt rearward overtravel from carbine buffer and no buffer spacer.  Note 9mm case stuck lengthwise in trigger pocket behind bolt catch.

[Image of bolt, overtravel]

Example of correct rearward bolt travel limit with extended-length buffer (4").

[Image of bolt, travel limited]

Last round bolt hold open (LRBHO) issues

- If the distance between the bolt catch and the fully-rearward bolt face was customized with buffer spacers to make it as small as possible, the bolt catch may not have time to move into position before the bolt reverses direction and is pushed forward over the catch. The bolt may need a little more rearward travel.
- The magazine adapter may sitting too low in the magazine well, or it may be holding the magazine too low, preventing the magazine from activating the bolt catch.
- Some manufacturer’s magazines may not lift the bolt catch enough to hold the bolt open.
- See “Bolt rearward over-travel”
- ASC or C-Products magazines, see HERE

Empty case ejection issues

- Inconsistent ejection pattern
- General ejection failures
- Empty cartridge case left in the action, causing feed jams
- Empty cartridge case left under extractor after the last round is fired and bolt holds open
- Stovepipe jams
- Fired cartridge case is crushed between the bolt and front of the ejection port

*** Ejector misalignment is very common with blowback 9mm. ***

- The ejector groove in the bolt exposes the ejector to a very small part of the outer edge of the cartridge case rim. If the ejector is out of place, it may miss the edge of the cartridge rim entirely, or just barely clip the rim. This may cause the cartridge case to remain in the action instead of ejecting, or may cause inconsistent ejection.
- To see the ejector's relationship to the bolt, look through the ejection port as the charging handle is slowly pulled back. There is a cutout in the lower left (bolt's left) bolt face. The tip of the ejector (the tip is usually rectangle-shaped) will become visible in that cut as the bolt is drawn backwards. The ejector tip should appear to fit snug in the corner of the cutout.

A properly aligned ejector:

[Image of bolt face and ejector tip]

- The most common recommended solution is to adjust the ejector by gently bending so that it rides as high (toward the top) and right (toward the bolt’s right) as possible in the corner of the bolt ejector groove, just barely missing/touching the bolt during bolt travel.
- Some ejectors are held in place with set screws that allow vertical adjustment of the ejector.
- Some manufacturers (e.g. Stern Defense) use hardened steel ejectors that may break if bending is attempted.
- The ejector tip face should also be smooth, flat, and square (parallel) to the bolt face. Angles, curves, rough spots, etc. may prevent the case from ejecting straight out the port.

- The cartridge case may be slipping out of position before ejection. The extractor may have debris buildup under the claw, insufficient tension, or may be damaged.
- A 9mm bolt that has the larger 5.56 extractor (e.g. Brownells, Faxon) (see picture above) may help control and align the empty case better for more positive ejection.
- An extra-power or .308 recoil spring pushes the bolt forward faster and harder than a regular spring, which could trap the fired cartridge cases in the ejection port before they have time to eject. This often results in a stovepipe jam.
- Using a Colt-style large plastic gas deflector (often mistakenly called a "shell deflector"; uses cut down dust cover) may cause the empty cartridge to bounce off the gas deflector and back into the action.

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(Edited to add new information)

This information is intended solely for entertainment purposes only. Do not perform any action based on any of the following information. The accuracy of this information is not guaranteed or warranted.

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Trigger/hammer/fire control group issues

- Trigger reset issues
- “Dead” trigger
- Hammer follow
- Light primer strikes
- Doubles/triples/runaway

- Trigger group parts may be broken or damaged.
- The AR 9mm bolts are not cut the same as 5.56 bolt carriers, so aftermarket trigger sets and precision triggers designed for 5.56mm may not work correctly in a 9mm AR setup. Some trigger sets are specified by the manufacturer as appropriate for the 9mm AR.
- Reduced power hammer and trigger springs have, under certain circumstances, been reported to occasionally cause “doubling”, where multiple shots are fired with a single trigger pull.
- Some users have reported that a 308 recoil spring caused doubles/triples. 308 recoil springs may not allow enough bolt dwell time for proper trigger reset.
- If the hammer has a pronounced “tail”, it may be striking the disconnector during reset, causing hammer follow.
- The disconnector spring may be weak, requiring replacement.
- Standard “mil-spec”-type semi-auto fire control groups seem to provide the greatest reliability.
- Also, see “Bolt bounce” for more.
- Also see “Franklin Armory BFSIII in a 9mm AR” for more.

Bolt bounce

- “Dead” trigger
- Hammer follow
- Light primer strikes
- Bulged cases/ruptured cases/out of battery discharge

- Symptoms may only appear during fast double-taps or rapid fire strings.
- Bolt bounce happens when, after firing a round, the bolt returns to battery and “bounces” off of the chamber face, partially extracting the chambered cartridge in the process. The bolt then slides forward again under recoil spring pressure. The bounce happens too fast for the naked eye to see.
- If bolt bounce is combined with fully automatic fire, fast triggers (e.g. Franklin BFS III, Fostech Echo, precision competition triggers), or simply a very quick trigger finger (rapid fire), the hammer may be coming forward for the next shot just as the bolt bounces backwards. This may cause what appears to be hammer-follow or trigger reset issues if the hammer comes to rest behind the bolt, or it may cause an out-of-battery discharge of the cartridge.
- Bolt bounce appears to be more likely when a solid buffer is used.
- A buffer with internal sliding weights helps prevent bolt bounce by acting like a “dead-blow hammer”. When the bolt closes, the weights inside the buffer slide forward a fraction of a second later and help cancel out the rearward “bounce”.
- See “Bulged or ruptured fired cartridge cases” for more.
- Also see “Franklin Armory BFSIII in a 9mm AR” for more.

General feeding issues

- Cartridge jams into top of the chamber at an angle
- Half-moon crease (smile) on bullet that failed to chamber
- Cartridge jams into barrel extension
- Cartridge jams into/below feed ramp
- Bolt “hesitates” during chambering
- FMJ feeds, but hollow point or truncated-cone bullets cause jams

- The feed cone may not be optimal for reliable feeding. Most barrels are still made with the original 1980's feed cone design, which is very narrow and was designed for feeding military FMJ ball ammo only. The feed cone can be re-profiled for more reliable feeding (Macon Armory can do this work, as well as others), or the barrel can be completely replaced with one that already has a generous feed cone profile (e.g. Macon Armory barrels, Taccom barrels).

A Macon Armory feed cone:

[Image of chamber feed cone]


- Polishing the feed cone may prevent cartridges from hanging up on their way into the chamber.
- An extra-power or 308 recoil spring may cause the bolt to move forward too quickly for the cartridge to get into proper position while feeding, causing nosedives and bullet setback.
- Magazines from a different manufacturer may feed better.
- After-market magazine extensions may cause feeding issues.
- The feed ramp may need to be raised or lowered, if it is adjustable.
- The magazine height in the receiver may need to be adjusted. Sometimes this can be done with a different magazine catch.
- The chamber face area may have a buildup of residue and requires cleaning.
- Different types of bullet ogive and different weight bullets may feed better.
- If the setup uses a 9mm magazine adapter, it may not be installed correctly or may be holding the magazine at the wrong height.
- The ejector may be riding too high, or may be pressing against the bolt during travel, slowing the bolt.

Last round feed issues

- Some magazines may have a sharp edge on the front of the follower that gets caught under the feed ramp when presenting the final cartridge in the magazine. As a result, the last cartridge may be too low in the magazine to feed properly, causing a jam. Beveling the front edge of the follower may fix this problem, allowing the follower to slip past the bottom edge of the feed ramp.
- ASC or C-Products magazines, see HERE

Bolt drags on magazine feed lips

- Some magazines may be out-of-spec, or just won't work with a particular setup.
- If using a magazine adapter, it may be installed too high in the magazine well, or it may have been pushed up due to forceful insertion of the magazines. Over-insertion features should be properly adjusted and functioning.
-  9mm AR bolts may be magazine-specific (e.g. Colt only), or a hybrid design allowing the use of multiple magazine types (e.g. Colt/Glock hybrid). The bolt may be cut incorrectly for the particular magazines or adapter being used. Bolt and adapter manufacturers may provide compatibility information.

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This is outstanding. Great information. Thank you very much for taking the time to share this.

This should be pinned.

wish this thread was available years ago!!! nicely done.

fyi re LAW folder adapter -

stern's SD BU9 9mm bolt carrier has an easily removable weight that is specifically designed to be replaced with a LAW extension.

best of all, it weighs almost exactly what the extension weighs, so no need to change buffer weights.

other pluses: "It functions with Colt SMG, Glock magazines" and "caters to single stage triggers with short resets.  It also allows use with the JP silent capture spring. This bolt is compatible with ... the Echo trigger."

Thank you, everyone, for the positive feedback. This multi-part post represents several months of work trying to get my first AR9 build working. I learned a lot in the process, and was able to complete a second build with absolutely no problems as a result.

Nearly all the information here was found by digging through old posts, where individual members helped out people who were having problems with their builds. A big thank you goes to everyone who has ever taken the time to post a suggestion or share your knowledge to help.

Great info.  I learned the hard way on a few issues.

1. Some AR15 drop-in triggers are not compatible with blowback 9mm.  Use drop in triggers specifically designed for 9mm.
2. Stock 5.5 oz buffers are too light in many guns.
3. LRBHO can be very sensitive to different magazine models.  Not all magazine followers activate the LRBHO at the same follower height.
4. LRBHO may need adjustment to not hold open too early or not at all.
5. Some magazine releases need internal beveling to ease mag release.

Droppoint... to add to your buffer weight data...

for a rifle length buffer

filled with #7 1/2 lead shot.... 6.0 oz
1/2" brass slug..................... 6.2
5/8" brass slug..................... 7.0
solid pour lead.....................  9.0

Have you had any experience with the Blitzkreig / KynSHOT hydraulic buffers ? Said to increase shock absorption to where they run very smoothly.

https://www.blitzkriegcomponents.com/ProductDetails.asp?ProductCode=RB5015HD

I'd like to do a Colt DOE clone on a MG registered cast  EA15  lower receiver but am terrified of cracking it.

Will use drop in mag block so as not to mark  receiver and take out the last shot bolt hold open lever.

I've spent a lot of time and money tweaking the Colt pattern 9mm straight blowbacks for full auto SMG matches so I can appreciate the compilation of all this data.

However, not trying to be a jackass but just about all the details you addressed are no longer issues with the CMMG RDB (radially delayed blowback) configuration.   Just a BCG and Barrel swap and use all standard 5.56 parts if going with the Endomags....no spacers, quarters, special fire control to handle the shock, no special buffers etc.   Truly just an upper swap.

That is all I run now....the Kynshot RB5007 (basically an A5 length hydraulic), A5 length tube and Tubb flat spring.

I use that in my 9mm CMMG RDB as well as my 5.56 uppers.

Detailed all this here: http://c3junkie.com/?page_id=538
and here: http://c3junkie.com/?page_id=654

Subscribed

Lots if good info in this thread. I am about ready to take the 9mm plunge. I have a Spikes Glock magazine lower, and a little hesitant on where to go.

I am thinking I'd like to do a side charger upper with either 8-10" or a 16" barrel. The charge handle could be on the right side or the left side of the weapon. Has anyone used a side charging upper on a Spike Glock lower and had the LRBHO successfully work? I want to minimize the parts buying and compatability issues with non-like branded parts.

Thanks for any info in advance.

My second 9mm blowback build is with a Spikes Glock magazine lower. The last round bolt hold open mechanism looks like it's made out of shaped thick steel wire but it works perfectly with both Glock and MagPul magazines. I used a standard Aero precision upper, but I don't think it would really matter whether it is a side charging or rear charging upper for the last round bolt hold open to work. I could be wrong.

Thanks!

No worries! All the information I provided is really focused on simple blowback systems. Mechanically delayed systems (radial, rollers, etc.) usually provide a superior shooting experience, but also tend to be more expensive, as should be expected. I believe the appeal of the simple blowback system is generally the lower price of the parts.

I would expect a blowback 9mm upper, properly ramped 9mm bolt, Endomags, and 9mm extended buffer could be priced out for a lot less than the comparable CMMG parts, with swapping the buffer as the only additional step for converting a mil-spec 5.56 AR to 9mm. That being said, the CMMG option is very nice, and would probably be a good way to go if someone can afford it.

Just the info I needed.....Thanks!!,

Great info! Thanks for sharing OP

Great info OP!
Getting ready to getting my toe wet in the AR9 platform. Any link rating the kits / uppers or what to steer clear of?
I’m planning on using Endomags and standard lower.

I'm not aware of any organized ratings of uppers or kits, but I can tell you what worked for me.

Both of my builds (Colt and Glock) use Macon Armory barrels (one is actually from another mfgr, but fixed by Macon Armory) with Rudy's 'guaranteed to feed' feed cone machining.  Both use Brownells branded 9mm bolts.

The Glock fed one has an AIM SSTAT trigger, uses a standard recoil spring, and a 10 oz extended KAK deadblow buffer. 5.5 barrel. Spikes Glock lower.

The Colt fed one has a binary trigger, a + power recoil spring (not exactly sure, seems to be about +25%) to help keep the binary from outrunning the bolt, and one of Macon Armory's 7.5 oz extended deadblow buffers. 4.5 barrel. Anderson lower, KAK Colt magwell adapter. Took it out 2 weeks ago and put 400 rounds (Federal AE 115 grain) through it without a problem.

Both have been running great so far.

I appreciate all the info, collected through sometimes less than satisfactory experience. I just completed a 9mm build. It has an AeroPrecision upper and lower, with Stern Defense major parts, the barrel, bolt and mag adapter for SIG 320 mags. It also has an Odin Works 9mm heavy buffer. I have only been able to take it out once so far. With the buffer weighing 6.2 oz, it worked perfectly. When I got back I switched around the buffer weights to get it up to the max of 7.4 oz. and I may put in a spacer in the extension to reduce bolt overtravel. I will take parts and tools to the range so I can make mods and try them immediately, but it did work fine the first time. I may be able to tune it to work even better.
Now if this Wuflu panic will subside we can get back to the important things.

what are people using to plug the hole in the upper for the gas tube?

The hole for the gas tube in an upper used for a 9mm blowback build is like your belly button, it doesn't serve a purpose any more, and it doesn't hurt anything being there. Just ignore it.

(Not a perfect analogy, but close enough...)

Every single ar9 bcg with the ar style extractor I've handled has had this plug (4 or 5 total handled). At this point, I think it's safe to assume you will get one with s removable plug if you buy from a reputable mfg.

OP, great info. I suggest you add KVPs 9mm buffers to your buffer list. They have 7.5 and 10oz buffers at the correct extended length for pccs. I'm using the 7.5 with their 9mm/308 spring and it works like a dream in my 5.5" build.

Just a quick note that I have been keeping this set of posts updated with new information, as long as I can fit it in the space available.  I should have reserved a couple more post spaces after my originals!

Excellent job, OP.  Thanks to the Mods for pinning this in the "important pistol caliber Threads!"  I've received all my parts to assemble my first 9mm blowback PCC.  I would have done this much sooner if the above info had been avail back in the day.  It was astonishing to me reading about the numerous broken parts, feed / extract failures, trying numerous parts to make one thing work right.  I just didn't want that headache.    Very nice to have the info published in one spot.  Again - nice job!!

Nice job Droppoint!

Thank you! Hope it helps keep some new builders from tearing their hair out from frustration...mine is still growing back...

Added some new buffers to the buffer list.

Wow....the most comprehensive fact sheets I’ve ever seen.

Well done Sir

Thanks OP!

Was having problems with a new build.  Empty cases would not reliably eject and many were getting caught up against the next round trying to feed.  Read what you said about the ejector and adjusted it per instructions.  Immediate improvement but not 100%.  Got more aggressive with bending the ejector and think I'm GTG.

I'm glad it helped! Ejection issues are a frequent problem with home builds (and some factory builds). It's such a minor thing to fix, but it causes so much frustration.

I was issued a Colt M635 for fifteen years and I loved it.

I found that two things were key to keeping it running.

1.  Lots of oil.
2.  If I every had malfunction (usually a double feed), I marked that magazine and usually it would happen again with the same mag, and I would just throw it away.  The magazines seemed to be the only weak link and one the lips got bent even slightly, it couldn't  be trusted.

Interesting! Thanks for the info.  Can you describe the double feeds in detail? Was it always 1 live round and 1 fired case, or something else?

Very generous OP. Thanks.

This should be a sticky.

Made a bunch of little edits and cleanup.  Added a couple of buffers to the list.

Added a section "parts to avoid".

Any additional input for this list would be appreciated!

Added some updates and added pictures to show proper ejector alignment and the feed cone.

Here are some tungsten weights That can be used to increase the weight of a buffer that is too light.
weights

Wow, wish I had found this earlier! Just completed a build and took it to the range, had some feed issues. Thanks to your post, I bought a scale and weighed the PSA 9mm buffer I got as part of their 9mm parts kit, and yep, they sent me the wrong (2.9 oz) buffer! I was wondering why the thing kicked so hard. To PSA's credit, they are sending out the correct parts as fast as they can, given the COVID/election buying surge. Next I looked at the ejector. 5D Tactical lower matched to a Foxtrot Mike upper. Yep, ejector was riding too low. That was an easy enough fix since the 5D ejector is held in by two allen screws. While 5D could provide a bit of guidance on assembly of their lowers with the lower, their tech support was solid in referring me to a post on Frontier Arms' site about adjusting ejector height properly. Looking forward to trying mine out again as soon as the new buffer gets here. Thanks again for your posts!

Woohoo! Another success story!  Glad it helped.

Added a section on commercial buffer spacers, and made various minor corrections and clarifications.

Droppoint, you inspired me to geek out over buffers and buffer springs!

I've tried out three buffers and three springs in my gun before finding a combo that felt good and fed reliably.

Buffers:

PSA "AR-9" which was the original they sent me in the PCC kit by mistake, it's an AR-15 buffer, 2.9 oz, 3.25" OAL, 0.25 face to land, sliding weight, mentioned it above
PSA AR-9 5.5 oz, 3.26" overall length (OAL), 0.25" from buffer face to spring land (where the top coil rests, you'll see why this matters in a moment), no sliding weight
Guntec AR-9 8.3 oz, 3.87" OAL, 0.806" face to land, and 0.782" diameter under the spring, which made it a very tight fit on the spring. I'd expect friction to contribute to spring force during firing. No Sliding weight.
Macon Armory Deadblow 7.5 oz, 4" OAL, 1.45" face to land, uses loose shot for weight like a deadblow hammer


Springs:

PSA "AR-9" from the original kit, splits the difference between my AR-15 and new PSA AR-9 springs
AR-15 that I measured for reference
JP AR 308 Carbine spring, as recommended by Foxtrot Mike
PSA "this time it's really an AR-9" spring

Data in the next post, since I'm still a noob and restricted in word count

OK, here's where it gets interesting. I'm a mechanical engineer, so springs are something I understand, but if you don't, I'd recommend David Tubbs' site, which has more than you want to know. I set out to measure my spring force to get a relative feel for what was going on with my gun, and just because I was curious. I used a decent grade, but uncalibrated, peak read push scale to measure spring force. The two things you need to know about any spring are its spring constant, k, measured in lbf/in, and its free length. It was easiest for me to use the length of the spring that stuck out of the buffer tube with the buffer removed from the gun, so I used that instead of free length.

1st PSA Spring: 3.78" Free length from buffer tube to top of spring, K value of 1.65 lbf/in
2nd PSA Spring: 4" Free length, K value of 1.65 lbf/in, so it's the same spring, but a bit longer
AR-15 Spring: 3.625" free length, K value of 1.67 lbf/in
JP AR 308 Carbine: 4.53" free length, K value of 1.49 lbf/in

If you know your buffer face to land dimension, you can now calculate the force when the buffer is in its rest position, or any other position along its travel. I measured mine with the push scale to derive K in the first place, so this was all a check. Next post, as I'm up against the Noob character limit.....

Where you start to see some variability is not so much with the springs as it is with the buffers, especially because that face to land dimension changes spring compression. I haven't learned how to post images here yet, so I can't post the complete matrices of springs and buffers. I measured force with the buffer in the rest position and at 1" and 3" of travel. I took the additional measurements as much for error control as for info, but they do help visualize how things change during the BCC stroke.

With the PSA buffers, the force at rest ranges from 5.81 lbf with the AR-15 spring to 6.88 lbf with the JP 308 spring. The New PSA spring splits these two at 6.5 lbf. At 3" of travel, the forces are 10.94 lbf, 11.38 lbf, and 11.5 lbf for AR-15, JP and PSA AR-9, respectively. The PSA's steeper slope (its K value) has overcome the JP's greater length of compression but shallower slope at 3".

Moving to the Guntec and Macon Armory buffers is where you start to see forces go up. Guntec is 6.74 lbf at rest with the AR-15 spring, 7.7 with the JP spring, and 7.4 with the new PSA AR-9 spring. At 3" of travel, the AR-15 spring is 12.17 lbf, JP is 12.2, PSA #2 is 12.4.

Macon Armory is 7.81 with the AR-15 spring, 8.67 with the JP, and 8.47 with PSA #2. At 3" travel, the numbers are 12.94, 13.17 and 13.55, respectively.

I expected bigger differences, but the Macon buffer with the JP spring feels noticeably nicer than the other options, though I have not tried the PSA #2 combo because the buffer is so short. If I ran a spacer for it, the forces would go up by the length of spacer x spring constant.

Lots of good reference material here. Thanks OP.
I seem to be lucky with my setup.

I originally bought a complete 9mm upper kit made by Yankee Hill.
It came with a ramped bolt.  A YHM marked 9mm buffer.
A hahn magwell adapter.  Colt magazine.
Basic 16 barrel. ztube handguard.  Fake gas block.

It has never malfunctioned. Maybe 1500 rds so not a lot per se.
All ball ammo.  
I swapped the barrel for a cheap 8" BCA barrel.
It has pretty much zero feed ramp.
And I am using 1 endo mag without an adapter.
Damage industries enhanced buffer spring.  
RTB enhanced trigger.


Still zero malfunctions.

Interesting analysis!  I've found that a regular carbine spring works just fine, and with the extra heavy weights the recoil forces are spread out over a longer time, resulting in less perceived recoil.  I could try to tune it further by messing with the springs, but the change in perceived recoil was so great when switching to 11 oz buffers, I dont think the difference would be very noticable.

Sounds like you've got a keeper. If the feed angle and mag height align the cartridge with the chamber, it should feed fine with ball ammo. Problems are often exposed when trying to feed truncated cone or large cavity hollow points like HSTs. If you'll only ever use ball ammo, you're good to go.

Mine are pressed into duty for home/personal defense and benefited from Macon Armory's feed cone machining.

What Drop said. Mine will feed the cheapest ball you can find all day, and most defense rounds too. It can't get past more than a couple Remington JHP HTP's without a misfeed. Those have a very blunt nose. Someday I need to figure out how to post pics on here.....

Added short section on barrel length.  No great revelations; just consolidating information.