Background on this is that I have an AR9 SBR that I’m considering converting to 5.56 due to issues that come with 9mm blowback in the AR platform.

I’ve been doing some research and I’m seeing that 10.5” SBRs have a reputation for breaking bolts. I can understand why with the short dwell time requiring the gas system to operate at higher pressure.

Does going to 11.5” or 12.5” over 10.5” make a significant difference for the life of a bolt?

It’s important to keep in mind that a decade and a half ago most short barrels were getting the same gas port sizes as 16” carbines. 223 ammo was more common than 5.56 at that time so additionally they were usually oversized to begin with.

Another factor are buffers. Back then heavy buffers were considered snake oil by many and were not common place at all. The idea of tuning guns with gas ports and buffers would have gotten you called names. There are many here that think replacing a buffer is somehow a negative compromise even to this day, despite the AR being built for it.

Throw steel case in the mix which doesn’t properly retract after firing, the lack of high pressure and magnetic particle testing, and broken bolts were a LOT more common in the old days across the board.

A properly set up short 5.56 gun will run just fine with minimal extra wear and tear. Even if you got 10k rounds instead of 15k, you’re talking about thousands of dollars difference in ammo. An extra bolt can be had for $100-$200, and that’s assuming a lot on round counts. (I have a 10.3 with around 11k on it with no failures or malfunctions aside from some Hex mags.)

The AR15 is ALL about balance. If the bolt is unlocking when pressure drops to the needed level, it doesn’t care if it’s a 10” barrel or a 24”.

Good info. I’d heard that Mk18s break bolts at 3-5k rounds but they’re kind of old at this point and I’d presume they just overgassed them to make them work.


Assume I wanted to go 11.5 or 12.5. What size gas port should I have, what gas system length, and what buffer should I be looking at?

I wouldn’t expect more than 7k out of a barrel and bolt in 10.5 unless its materials or process are different than GI.

11.5, carbine or carbine plus 1, .070 or smaller in the first case, H2 or H3.

12.5 carbine plus 1 or mid, the latter if you have a can.

Yes longer barrels and gas systems have longer component life.

What is carbine plus 1?

I sheared a lug off a bolt at around 12k rounds on a 10.3” gun. Ironically the rifle still functioned(ish) with a missing lug. Replaced the bolt and ran it another 5-6k rounds before that rifle was replaced.

My current rifle is an 11.5” gun and I’m right around the 10k round mark. No issues so far

Both rifles are/were shot suppressed probably 2/3rds of the time. I have/had to do the “full-auto qualification” every quarter. I’d estimate 2-3k rounds were “controlled bursts”

I feel 10k rounds to be more than acceptable, 98% of people will never see that round count ever.

A nonstandard longer gas system.

I have an 11.5" with about 7k through it.  0 issues and runs like a top.  It's my go to rifle.   I run the A5 buffer system and I think this makes a big improvement over mill spec.

Suppressed or no?

My 10.3" has a .063 gas port and will function with hot 5.56 ammo and a carbine buffer without a suppressor. However, it wears a YHM Turbo T2 and has a Super 42 w/H3 buffer 99.9% of the time.

My 12.5" has a .0635 BRT gas tube (.0615 gas port equivalent) and wears a YHM Turbo K and has a Super 42 w/H2. I need to test it unsuppressed for function and locked, but I don't know why I'd shoot it without the can.

adjustable/tuned gas helps reduce the abuse.

My 10.5 is super soft to shoot.

Not suppressed to start with.

Then I'd shop for a barrel with a .070 gas port at the largest for 10.3", .068 for 11.5", and .066 for 12.5", assuming carbine length gas. Those are all known to run well and be soft shooters with quality parts. However, if you shoot weak steel cased ammo those ports may be a bit small.

I wouldn't worry about it. I haven't broken a bolt yet and I have quite a few shorties.

Keep a spare BCG if you're worried about it.

^^^^^

That's pretty fair.

Only reason I care much about gas system length and port sizes is to minimize gas to face without adding the adj gas block as a potential failure point. Softer shooting and wearing is just a bonus.

I can tell you one I just pin gauged: 12.5" BRT with their in-between carbine and mid length has a 0.073" gas port, and with an H2 buffer it will not cycle IMI 5.56 unsuppressed.

Attached File


I need to break a few others down and gauge them too.

https://www.ar15.com/forums/ar-15/A-10-5-at-11-000-rounds/122-782686/

Bolt running fine still.  Did have to replace an ejector spring, that got tore up from a life of high rate sustained fire.  Barrel is almost shot and won't stabilize 73 gr bullets anymore; but still runs 55 ball FMJ just fine.  But bolt is fine.

If I can find a cheap 11.5 Barrel, I'll swap mine out - but didn't find a good black-Friday sale.  Ah well.  I suppose I could try to ask PSA to replace it

Wow that's an awesome thread, at first I was wondering how I missed it then I saw the pistols subforum. Dang.

I just skimmed at first, now going back to really read. First thought is how nice to see a known round count and how everything is actually holding up. I bet 99% of AR owners never put 11k through one gun in their entire lives.

Wild. Do we know who makes these barrels?

I would think a 556 barrel and bolt would wear faster than a 9mm barrel/bolt (gas erosion, more moving parts, etc.)… but I don’t know.

I am curious as to what issues with the AR9 you are worried so much about that it makes you want to switch to 223/556….

1. The AR9 is a heavy pig and, unfortunately, it kind of has to be due to direct blowback operation, despite having a 5.5 inch barrel.  With a fixed stock it would make an excellent war club in addition to being an SBR.  The balance isn't great...just doesn't feel right especially for shooting standing unsupported.
2. Recoil is odd. Recoil energy is probably similar to 5.56 but there's something snappy and nasty about it, like the recoil impulse is sharper / more sudden. It's just enough to make it unpleasant to shoot if you're putting a lot of rounds downrange
3. Reliability.  Using a magwell insert and ASC mags. Probably having one malfunction per 250 rounds or so. With the CPD mags it's one malfunction per magazine.
4. It operates more reliably if the bolt is running at speed, so it has a rather stout recoil spring. This makes charging more difficult for my wife. I don't care for it personally but I can deal.

Right now the only clear advantage it has is cheaper ammo.  I can handload 5.56 for around 0.27 / round.  I can handload 9mm for around 0.18 / round. This might be an argument to convert to 22lr, though I've already got semi auto 22s.

I'd say the poor balance mentioned in #1 is probably my biggest gripe of all these.  There's something about the weight + balance that makes it kind of crap for standing unsupported shooting.  I can hold steadier unsupported with my 20" thin barrel profiled AR. It either needs to be lighter overall or it needs to be nose heavier. Of course anything with an AR9 bolt in the receiver and 5.5 inch barrel in front of it is going to be very not nose heavy.

Direct blowback 9mm in an AR is a fine idea that seems to have some rather significant implementation problems.

I can agree that they can be heavy.  I have a 633, 635, and another 9mm AR with a 8" barrel, the 635 and the other are on the heavier side.  The 633 (DOE) is pretty sweet.

I find the recoil to be much better when keeping my bolt and buffer in the 22.5-23 oz area and using sliding weight buffers or the KAK K-Spec buffer (I REALLY Like this buffer).  To me its more of a push than anything.  Kinda feels like a 556 with a rifle length gas system and not the sharpness of a carbine.

Magwell inserts... two of mine use them, the KAK insert's to be exact, and I dimpled the inside of the well where the two set screws rest.  While neither have given me problems, it is a worry of mine in regards to long term use.  So I can understand this concern.

But reliability, I have had zero issues with my 3.  I find them fun to shoot (I use Colt and metalform mags) and like you said, they are cheap to shoot (I need to get into reloading).  Honestly, i enjoy shooting them more than my 556 SBR's, but that may very well be me cringing at the $$ flying down the barrel with the 556 guns...

I enjoy mine and was curious what your direct concerns were, thanks for sharing them!

As for the feeding issues, what I'm seeing is that the cartridge is not controlled very well during feeding no surprise as the bolt doesn't have an extractor. It's mostly ok if I'm holding normally. If I have the rifle rolled one way or another, feed malfunction is more likely. Running the bolt at speed mitigates this somewhat, which is part of what concerns me about slowing the system down with a heavier buffer.

I have a Spinta Precision bolt and KVP 7.5 oz buffer. I think the weight on the bolt is 12 oz so I should be right at 19.5-20. I could be wrong but I have doubts about 2.5 oz more weight really making a big difference. Correction. I weighed my bolt and it comes in at 15.6 oz. I already have 23.1 oz of reciprocating mass.

The Maxim roller delay system is interesting but makes the gun hard to charge. My wife probably couldn't charge it with that system. The MEAN Arms system looks promising but they only sell it as a complete upper which I don't need and it's very $$$. The Scheel roller delay is more expensive than the Maxim and has less effect. CMMG's radial delay would be a good solution if it didn't eat ejector springs every 1k rounds.


I'm either going to wait for a viable mechanical delay system I can afford that works, go heavier, convert to a 12.5 5.56, or convert to 22lr.

I have one of those KVP 7.5’s also and you know it really makes the recoil feel much more sharp than the KAK buffers with sliding weights or the K-spec.  Might try a different buffer and see if it helps reduce the recoil.


I know that was really off your desired topic, but just a suggestion.

The rolling systems look interesting, but I like you, don’t see a viable one that does cost an arm and a leg or requires me to buy much more than I want/need.