I recently built an AR PCC 9mm.  After a lot of searching, I found many combinations for bolt and buffer weights that may or may not work for my particular setup. However, I also found that barrel length appears to be a significant part of the reliable-blowback-operation equation.

I'd like to call on the membership to help put together a database of functioning AR PCC barrel length, bolt weight, and buffer weight combinations. With your help this could be a fantastic resource for people building or troubleshooting their own AR PCC.  What worked for you???

Please follow this format:

Caliber:
Barrel length:
Bolt weight:
Buffer weight:
Additional details: (what works, what dosen't, suppressor use)

(Any other details as you would like to add)

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I'll kick things off.

Caliber: 9mm
Barrel length: 4.5"
Bolt weight: 14.2 oz.
Buffer weight: 5.6 oz.
Additional details: Works with 147gr., hotter loads, and with suppressor. 115gr light range loads cause occasional short strokes.

9mm Alpha Shooting Sports premium bolt with internal pinned weight removed, LAW folder plug inserted. Using carbine buffer spring with 13/16" elastomeric plug in the back of the buffer tube to reduce bolt travel.

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Here's some additional general information I compiled that may be useful:

A typical 9mm bolt weighs about 15.5 oz. The internal weight and pin weigh about 3.5 oz. leaving the bolt at about 12.0 oz. when removed. The LAW folding buffer tube bolt extension plug weighs about 2.2 oz.

Standard Colt 9mm buffer+bolt weight = 21.5 oz., however this appears to assume select fire, hot loads, and a 16" barrel.

Reserved...

Mine got archived

https://www.ar15.com/forums/AR-15/-ARCHIVED-THREAD-The-complete-9mm-AR-trigger-thread-What-works-what-doesn-t-work-database-Please-add-yours-/15-713015/?page=2

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Caliber: 9mm
Barrel length: 16” Rock River barrel
Bolt weight: Standard bolt from Rock River (later ramped by ADCO)
Buffer weight: Standard carbine buffer
Additional details: With the standard buffer, the gun chews up aluminum cases as it ejects them.  I haven’t tried my 5oz buffer in this setup yet, but I think it’ll help.

Caliber: 9mm
Barrel length: 10.5” from Spike’s
Bolt weight: Standard bolt from CMMG (came ramped)
Buffer weight: Standard carbine buffer
Additional details: Just like with the 16” upper, with the standard buffer, the short upper chews up aluminum cases as well.  I haven’t tried my 5oz buffer in this setup yet, but I think it’ll help this one too.

Note: The short upper runs great on either my standard (SBR’d) lower or my early model QC10 Glock lower.  I haven’t tried the 16” upper on the Glock lower yet.  I use a standard weight bolt from JSE that Rudy at Macon Armory did the Glock cut on for me.  I need more range time...

Caliber - 9mm
Barrel - 7.5” Quarter Circle 10
Bolt - Quarter Circle 10 Universal 9mm Bolt. Weight unkown.
Buffer - KynShot 9mm Hydraulic Buffer. 6 ounces
Spring - Wolff Extra Power Spring
Trigger- ALG Combat Trigger with 4.5 pound purple spring
Lower- Quarter Circle 10 9mm Colt
Upper- Quarter Circle 10 side charger
Spikes Tactical Barking Spider Compensator

Eats 115, 124 and 147 no problem.

I had to file the ejector of the QC10 lower to make it a straight line. The ejector angled down slightly and it was simple to file it flat. This corrected all failure to eject I was experiencing.

See how it’s slanted?
Attached File


Filed it flat
Attached File


Kicks like a .22 at most

Attached File


Caliber .45
Direct Impingement
Barrel - 11” Macon Armory
Buffer - Standard Carbine Buffer
Spring- Standard Buffer Spring
Bolt- Macon Armory .45 Bolt
Trigger- ALG Combat Trigger with 4.5 pound purple spring.
Upper- Angstadt Arms
Lower- Angstadt Arms Glock

Had to send the Angstadt Upper to Rudy at Macon Armory to slightly open up the ejection port. The Angstadt Arms ejection port was slightly too narrow for a .45 case to eject cleanly.

Kicks like a BB gun

Eats everything now.

Attached File

9mm Glock mag fed
Spinta blem bolt
16" NFA barrel
Lower was bought complete from PSA with a 6 position stock - per their CS, lower is built with standard milspec kit EXCEPT for 9mm buffer which is same as they sell
I put an A2 stock on it, using a oak dowel and a quarter to keep internal length of receiver extension the same as the 6 position tube, so I'm still using original carbine spring (wanted to keep the working bolt-buffer-spring combo the same on the new stock)

Runs great

Caliber: 9mm
Barrel length: 8.5" Faxon
Bolt weight: 14.9oz Faxon Gen 2
Buffer weight: 6.0z Blitzkrieg 9mm Hydraulic Buffer
Additional details: JP 308 Carbine Ground & Polished Spring

Almost 2k rounds through it without issue.

Caliber: 9mm
Barrel length: 7" -- Rock River Arms barrel
Bolt weight: 14.65-oz. -- Faxon Firearms PCC Gen.2 14.65-oz. (not sure where they get their claimed 14.9-oz. weight)
Buffer weight: 4.35-oz. -- Geissele Automatics Super 42 Buffer & Spring combo +1 tungsten weight making the buffer an H2
Total B&B weight: 19-oz.
Additional details: Gun ran fine with Geissele H1 buffer, added 1 weight to slow the cycle a little. Runs great. Still experimenting.

Note: I put this upper and the Geissele buffer/spring combo on/in one of my Transferrable MG lowers and it ran great in full auto. No issues. Still experimenting.
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Attached File

what doesn't work?

Geissele SSP trigger doesn't reset with most 9mm AR Colt-compatible bolts.  SSP trigger does reset with Foxtrot Mike FM-9 Colt-only 9mm AR bolt.

Colt-pattern Hahn dedicated magwell adapter
Caliber: 9mm
Metalform 32rd mags
Barrel length: 16" (Macon Armory enhanced feed cone)
Bolt weight: std (15.5oz)
Buffer weight: 6oz
Additional details: my Geissele SSP trigger did not reset with KVP, AIM, Faxon, Spinta bolt carriers.  However if you release the trigger, and pull the bolt back again with the charging handle (unfired round extracted from chamber, new round loaded), with the finger off the trigger, it cocks the hammer.  In comparing the pickup lug on the bottom of these bolts with the pickup lug of a mil-spec 5.56 BCG, they are about .040" shallower than the 5.56 carrier, consequently, they don't press the hammer down as far, and it doesn't engage with the disconnector.
I'm shooting strictly unsuppressed.

tried it with KVP, AIM, Faxon, and Spinta bolts.  all the same.  also tried a 2nd SSP trigger lower, same result, doesn't reset.  The bottom of the bolts I have is too shallow to push down enough to reset the SSP trigger, when compared to a regular AR15 BCG.  Guess the SSP needs that little extra nudge to reset that would normally be present with the AR15 BCG.
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What does work:
I purchased a Foxtrot Mike FM-9 Colt-only bolt and that has a different shape on the bottom of the pickup lug, the extra material there pushes the SSP hammer down far enough to reset, and still is within the "envelope" of the mil-spec BCG dimension.  So everything is good now.