I built a 9mm Oly Arm's 6.5" barreled upper for RR. It runs fine semi but I only get two maybe three round auto before the bolt closes on a bullet that didn't quite make it to the chamber. It cycles VERY fast too. Comparable to my factory M11/9.
This is a Glock mag well adapter gun. I'm using a Spikes 9mm buffer. What can I do to slow it down? I feel like that would fix a lot of the problem.

Heavier buffer.

Help me understand this - you have an Oly pattern upper with a Spikes BCG? I know diddly about Spikes stuff but assumed it to be the Colt pattern? AFAIK the Oly barrel extension is not compatible with a Colt bolt. What about the ejector system, Oly or Colt?

I run an Olympic 9mm upper on my RR with an Enidine buffer which smoothes it out and does reduce the ROF a bit.

What buffer and spring are you running?

That was a typo. The only thing in it that is Spikes is the buffer. They have that longer buffer specifically for 9mm.
The entire upper is Oly.
The buffer spring is a standard carbine spring.

So I thought it might be the buffer weight that was the issue. But since this buffer is specifically for 9mm I wasn't sure. This one is the Spikes ST-9x, it's 8ounces. Now the issue is the buffers to replace it with are 10 and 12 ounces. None of them are cheap and I have no clue which one I would need. Are there heavier options out there? Any recommendations?

Pictures always help.

I did some reading on the Enidibe buffer but I couldn't find what they weigh. Does the hydrolic compensate for the weight? Is there different weights? I see there is a hydrolic 9mm AR buffer from Blitzkrieg. Anyone have experience with that one?

I'm just sceptical of all my options because of the price. I don't want to make the wrong purchase again.

The extra-long Spikes 9mm buffer is part of what's upping your cyclic rate, by reducing the stroke length. Simply going to a standard-length H3 may go a long way towards evening out your cycle in full auto.

ETA: My Oly 9mm upper runs fine with a standard (not heavy) carbine buffer at about 1250rpm cyclic, when mounted on one of my RRs.

Ok, everything I had read said you should use the longer buffer for 9mm. That does make sense though. I'll give that a shot.
I have a 5.3 ounce buffer I can try.
Depending on how it runs maybe I'll try this buffer in it: Slashes heavy buffer

There are pros and cons to running the longer 9MM dedicated buffers and/or the 9MM spacer in the bottom of your buffer tube.

The pros to the 9MM buffer or spacer is that the bolt doesnt get a running start at the last round bolt hold open lever.  The main reason that 9MM guns break LRBHO levers is because the overall length of a 9MM Bolt/Carrier is shorter than 5.56 variant.  So when the 9MM bolt bottoms out in the buffer tube there is a sizeable space between the front lip of the bolt/carrier and the LRBHO level.   The bolt then gets a .5 to .75" running start at the LRBHO lever where in the 5.56 version there should be much less than space.

The longer 9MM buffer or spacer takes up this slack and keeps the bolt from ramming into the LRBHO lever.

On top of breaking LRBHO levers the repeated ramming can also damage your lower.  My person 9MM SBR sear host lower shows obvious signs of metal deformation due to this issue.

The big downside to  these spacers or longer buffers is that they obviously speed up the cyclic rate on auto similar to how a MAC-JAC works.  I notice a sizeable increase in ROF with the 9MM buffer in place.

All that said if you are running a setup that doesnt actuate the LRBHO lever (like a glock system) than I dont personally see the point of the longer buffers or spacers.  You take all the downside of the increase in cyclic rate with none of the upside.

Personally for me I go back and forth.

When I am shooting my 9MM SBR host with a sear in it, I pull the 9MM spacer and enjoy the slower cyclic rate on auto.  I will also try to not fully run the gun dry if I can help it.  I also have a couple mags with the LRBHO tab on the follower disabled.

When I am running the same gun in IDPA/Carbine/3-Gun type events I pull the sear and the buffer since I get the benefit of protecting the LRBHO and lower since I am utilizing the LRBHO mech alot more.


Since you are running a glock mag setup and need to slow the rate down, I would get a standard length steel bodied with 3 tungsten weight bolt, like a Colt X buffer or one of the many other mechanically identical commercial variations available.

Are you saying just shooting 9mm will damage the lower, or shooting 9mm with the wrong buffer?

Also I did a quick search. I don't see any Colt X buffers out there on the market. Does the 8.5 ounce Slash buffer sound like it would be an issue? Its the same size as a standard buffer.

This video has some info that may help:

Yes and no.

Heavy usage of a Colt 9MM pattern system with a standard carbine buffer (of any weight) ....assuming your regularly engage the Last Round Bolt Hold Open (LRBHO) feature will in my experience/opinion damage your lower over time.

This isn't really due to the weight of the buffer but the fact that the 9mm bolt/carrier (as it is really one piece) is shorter than its 5.56 counterpart.

See pic below side by side.



This difference in length when running a standard length carbine buffer results in the carrier getting a running start at the LRBHO.

Here is a pic of the difference of a 9MM bolt/carrier bottomed out vs. a 5.56 carrier in relationship to the LRBHO.



To add insult to injury the 9MM bolt/carrier weighs more than the 5.56 version and generally requires a heavier buffer to control the recoil impulse and ROF.  So more weight at ramming speed certainly doesnt help things.

So take a heavier bolt, plus a heavier buffer, and give it a 3/4"+ ramming head start into the LRBHO lever and you ultimately end up with broken LRBHO levers plus damaged lowers.  The bolt rams into the LRBHO lever, the LRBHO lever ultimately has to transfer that stress somewhere.  What I have experienced is the LRBHO lever rocks forward on the top end, while the lower portion rocks backward.  You end up with a divot in the lower.  See pic below with the cresent shaped outline of the LRBHO pocket and the annodizing that has been effectively knocked off.



So what can you do prevent potential damage to your lower when running 9MM.

1. Dont regularly engage the LRBHO  on a Colt pattern system.  Let the bolt slam home on an empty chamber.  Easy fix.....  Either use a system that isn't capable of actuating the LBRHO lever (like glock), use modded Uzi mags which have no LRBHO tab with a Colt pattern system, or mod the LRBHO tab on a Colt pattern mag so it doesnt push the LRBHO into the path of the bolt carrier when the mag runs dry

2. Use a 9MM spacer that takes up this room and effectively makes a 9MM bolt/carrier the same overall length as a 5.56 bolt carrier.  (see example below)



3. Use an extended 9MM specific buffer like the KAK industries buffer that effectively has the spacer built in.



The downside of options 2 and 3 is you are shortening the cyclic space of the bolt which results in a higher ROF.  

For the non-professional machinegun "enthusiast" the best option in my opinion to control ROF and prevent damage is to run a "heavy" but standard  length carbine buffer and not regularly engage the LRBHO feature.  The other option is just go into the arrangement knowing you are going to potentially fuck up your lower and make peace with it.  

I personally wouldnt use a Colt pattern setup and leverage the LRBHO functionality on a expensive m16 registered receiver. (Not to mention the potential FCG pin hole damage on any 9MM system when you eventually bust a set of FCG pins)  

On a registered SBR lower I use with a sear, I dont really worry if I damage the lower.  It is really more cosmetic than functional and a SBR lower has really no resale value damaged or not.


I dont have any experience with Slash buffer, but OEM Colt X buffers can be purchase from Ken Elmore @ Specialized Armament Warehouse.  

http://www.specializedarmament.com/ar-15-9mm-smg-carbine/buffer-assembly-9mm-smg-x/

Also the only FCG pins I have yet to break (knock on wood) on a 9MM setup are KNS anti-rotation pins.  I have even broken the stainless steel OEM Colt pins over the years.

Hope this help explain/clarify the previous post.

Anyone tried a Wolff XP hammer spring to slow the carrier slightly? Seems like it could extract a little energy from the rearward stroke without automatically adding it back in.

I think A5 buffers might be an option in standard carbine tubes. Whatever buffer your using make sure it has weights on the inside.

Interesting. I wonder how much it would cost to get a damaged receiver repaired.

A while back I was looking to build a 40 cal upper. I was warned against it because of exactly what your talking about. I had not heard that 9mm would do the same thing. I just figured 9mm must be fine because it's not uncommon to see 9mm M16's listed on subguns.
This has me thinking of selling this upper.

Repairs are something I could see doing on a Sendra or Oly and absolutely avoiding on a Colt.

I wonder if some of the damage could be mitigated by increasing hammer spring strength, decreasing main spring strength and using a heavier and longer buffer. Probably add KNS pins for good measure.

If I was forced to run a 9MM system on a registered receiver (Colt or otherwise) I wouldnt use the LRBHO and would use either KNS pins or DPMS antiwalk pins.


Every set of hammer pins I have broken always fails at the indent in the middle of the pin where the J-Spring from the hammer engages.

I dont think that the KNS pins are necessarily made of some super strong material vs. a normal stainless pins but they dont break as readily since the j-spring groove is much reduced in size on the KNS version.

The DPMS anti-walk pins have no j-spring groove at all so would seem to be a potentially even better option from a breakage perspective, but you give up the pin load distribution across all four holes via the KNS system.




I have actually talked to the owner at KNS about making some anti-walk hammer pins with no j-spring groove and he wasn't interested.  Although I have considered making a custom set for myself.

Seems that the KNS system which distributes the hammer pin load to all four holes and didnt have a j-spring groove cut in it would be the ideal setup for blow-back pistol uppers.

You could also make sure you are running a ramped bolt (which almost everybody does now-a-days), install an extra power hammer spring, and run lighter 9MM loads like Remington UMC.

I do have KNS pins in it and do used the stronger hammer spring.
Doesn't really matter though. I decided no more 9mm in my RR.

Just out of curiosity, as it seems like only a carbine length buffer or 9mm buffer are mentioned, but what if you use a rifle length buffer in a regular a1/a2 stock?  Does this help prevent wear on the lower?  Or does that not matter either?

Carbine and rifle buffers both have the same length of travel, though standard rifle buffers are heavier than standard carbine buffers. If the weights were identical (heavier carbine buffer), the only possible difference would be in the spring constant, and whether someone was using a spacer or stacking quarters to limit travel.

Rifle extension and buffer do nothing different from a heavy carbine buffer to address pin wear from the blowback recoil impulse being transmitted to the FCG pins.

I figured it couldn't be that simple, by using a rifle length buffer.  

So what about this.....using a Hahn upper drop in mag block that has no LRBHO feature, is that safer?
Shouldn't put any stress on the bolt catch, correct?

Yeah, long as the catch isn't getting hit by the bolt, there shouldn't be any stress on it.

Thanks, just bought a NIB colt m16a1 yesterday, don't want to mess it up, and was planning to shoot 9mm out of it.

Was this directed at me?  
If so, yea I'm good.  My last post was a statement, not a question.  I'm planning to shoot 9mm out of mine, and will take the necessary precautions.  Just sold a m10/9 also. So I'm not really interested in the mac platform.

Yea and what's even funnier is that the Preban colts are actually going for even less than that now! ~1200-1500ish

But I hope I didn't derail the OP, I figured my question went along with his question.

I have a few 9mm mags dedicated to use on full-auto where I've trimmed off the tab at the rear of the follower, so it doesn't actuate the bolt catch. Cheap modification that took about 10 minutes with an X-Acto knife. I don't worry about it as much when I'm using it on a (relatively) inexpensive SBR lower.

I have been running 9mm on a Vltor A5 receiver extension with the standard A5H2 buffer weight (5.33oz, just slightly less than a 9mm carbine buffer). I plan to get an H4 (+1.5oz) to see how it runs when I have more time to tinker.

Anyone with a colt x buffer?
Can you post the weight of that buffer?

ETA: found it, 7.9 oz if anyone is interested.

I'm also wondering, what is the reason for going with a steel buffer over the standard aluminum body when using it for 9mm?

Because steel is heavier than aluminum, for same sized objects. When it comes to running 9mm in an AR with an unlocked, "blowback" system, weight is both your friend and your enemy. The original AR bolt carrier is very light and relatively small, and the rifle's upper receiver group is constructed around this component. Due to those size constraints, it is a bit difficult to add substantial weight into the AR "cycling mass" (bolt/buffer) for blowback operation, so heavy materials come into play.

Automatic weapons designed from the ground-up for blowback operation at "normal" cyclic rates have big, heavy bolts. For example, pick up an UZI SMG, STEn, or M3 "Grease Gun" bolt; they are large, brick-heavy chunks of steel. Without the benefit of a locked breech, you need weight- lots of weight.

I seem to be the only one to do it this way, mostly likely because i have a sear and not a RR.
With shorter 9mm uppers especially unsuppressed the easy way for me to get 100% function is the wolf xp spring and an H buffer, no shims and just let it slam away on the bolt catch.

I do dial in the 9mm setups so they dont run as hard.......but once again tdias so who cares.


Sorry it doesn't help you OP, I was opposed to doing it "my way" at first....but the manufacturer of one of my crap 9mm uppers told me to do it this way.

If i had a RR i would just forgo the bolt catch all together.

I also agree with the other poster about doing 9mm though an uzi, Mac,mp5
Now that i have all 3 i doubt I'll ever shoot my 9mm ARs ever again
I hate 9mm ARs...if i could own a suppresser i might feel differently

Thanks for that info.  So I'm trying to decide on going with an h3 buffer (5.5 oz) or an x buffer(7.9).  Any advice?  I'm not utilizing lrbho so the spacer or extended buffer is out.