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Discussion Starter · #1 ·
I would have gone over to ar15.com and posted but would have got flamed there and figured someone here would know the answer to my question anyway.

I have a 10.5" SBR (short barrel rifle for those that may not know). My problem is this. I have probably run about 1500 rounds through it. I have had about 5 failure to feed. A couple of them happened in the first few hundred rounds while I was shooting. The bolt simply didn't go far enough back to load the next round, but far enough to eject the fired round and then the bolt closed completely giving the appearance that everything was fine. I figured these 2-3 FTF's in the first 500 rounds were just the gun breaking itself in.

Last week my 8 year old and some 12 year old scouts shot my SBR and 2 of them had FTF issues. This time though the bolt cycled far enough back to eject the fired casing but jam on the next round going forward and jam with the bolt about 1/2 way closed. I believe these FTF issues were because of the "little" guys shooting the gun and not being able to absord as much of the recoil. Even though the gun is pretty wimpy on the recoil the little guys can't absord as much of it so not all of the recoil goes into the bolt.

I belive the problem could be solved in 2 ways, but I really don't know. If I can get more pressure built up in the gas system that would help. I know there are some muzzle breaks that supposedly increase the pressure in the gun. The other thing I wondered about is a different buffer spring, like for a 9MM. I also wondered if it could be a magazine problem but haven't tracked close enough which mags it has happened on cause it has only been about 5 rounds out of 1500.

FBMG, Eukatea, you other AR experts out there, tell me how to solve my problem and what you think.
 

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Discussion Starter · #3 ·
FBMG Smithy said:
Take a look at the mags, the ammo (you might just have to use ammo with more punch) and let me know.
The first 400 rounds were American Eagle. The last 1100 rounds have been cheap Wolf. Interestingly the Wolf have only FTF on myy 8 year old and 2 of the 12 year old scouts. Never on an adult.

What specifically am I looking for on the mags? I have already inspected them and don't see anything wrong. They seem to be fine and there is nothing obviously wrong with them that I can see.
 

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look for any abviose damage. If nothing look out of place try shooting them to see if it happens on a spacific mag. If you find one toss it in the trash. If not then bring it in and we will have a look at it with no charge. If it needs some work we will go from there.

Note: stay away from steel cased Wolf ammo. You will get more and more failure to extract from them. It was build for brass casings.
 
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Do you know what kind of buffer is in there?

One problem with a 10.5" barrel is the gas pulse. A 10.5" barrel has the same length gas tube as a 14.5 inch barrel but 4" shorter after the gas port. To get them to run they usually have a large gas port so you get a hard but short burst of gas to cycle the gun.

I think you are correct in your evaluation of why it happened to you children more than it happened to you. There are two things that usually fix this; enlarging the gas port and a heavier buffer. A heavy buffer will carry more energy than a light and since you have a very short but strong gas pulse in your 10.5" barrel you want to generate and store as much energy in the buffer as you can so it has the energy to cycle back into battery. Check what buffer you have, if it is unmarked weight it. A standard carbine buffer weights 3.0 oz and is probably too light for a 10.5" barrel. If you have them or can find them try an H2 or heavier buffer. You could go so far as to get a 9mm buffer 9.5 oz. Some folks prefer the recoil of a heavy buffer; they find it milder than light buffer.

If you can't find an H2 or heavier buffer I have a few if you want to come all the way out to lake point to get it.
 

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Discussion Starter · #7 ·
Thanks Eukatea,

I am almost certain it has nothing to do with the Wolf ammo and the mags. I have run the exact same ammo using the exact same mags through my 16" barrell AR and never had the problem. So I can't belive it is the ammo or mags.

I have a standard carbine buffer, (I think). Whatever buffer comes with a standard off the shelf 6 position stock.

Do you think replacing the buffer alone would fix it? And exactly which one would you try first? Thanks.
 
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Edit: Joe beat me to it!!!!!!!!!!!!!!!!
Buffers are a loaded question. It is rare that a heavier buffer will cause a malfunction so you can probably go as heavy as you can find; if that is your want.

The heavier the buffer the longer the recoil event and the felt effect is less sever but cycle time is high. long cycle times are great for machine guns, I run a very heavy system in my select fire rifle, as it reduces rounds per second which lowers cost, eases muzzle control and trigger control. In a gun set up for fast target acquisition slow cyclic time has a disadvantage, it takes longer to get back on target; 3 gun matches are a great example, I run a as light a system as I can in my semi only rifles. But that is personal preference. As I said some folks like the recoil they get from a heavy system.

Standard buffers usually cost less than 20$ and come in a variety of choices:

3.0 standard carbine
3.8 H
4.6 H2
5.4 H3
8 XH (I've never seen an Xh buffer I guess they are rare)
5.5 9mm
6.3 9mm A
7.1 9mm B

There are a billion more custom or one off buffers out there but these are the ones that should be commonly available.

If you can spare the cash try a few, I would probably start with an H2, see how it feels then try something else but I think an H2 will cure your rifle. You can get fancy buffers as well MGI makes a really heavy buffer with a mechanical recoil/rate of fire dampener; Enidine makes a hydraulic one that is a good deal lighter than the MGI, they are both expensive 80+ bucks if memory serves.

If you don't want to lay down the cash for a couple buffers I have some you can use I even have an adjustable weight setup that can go as high as 14 oz and a place to test them.
 

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PW said:
Thanks guys.

What do you think about these muzzle breaks that supposedly increase the gas pressure and help SBR's?
I've never seen one used but if they stand up to their claims then they can't hurt. That's if the stand up to their claims.
 
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I have never seen one either. Of course any muzzle device is going to prolong the gas pulse I doubt they can raise gas pressure. According to Bernoulli's principal to raise gas pressure we need to decrease velocity, which means a constriction (not possible as a bullet needs to pass) or a cavity to decelerate the gas. This would raise pressure near the aperture of the cavity but not at the gas port so no benefit in gas pressure would be gained (this is all based on my poor understanding of aerodynamics which is poor at best).

I wrote all that gibberish to cover something we don't want anyway. Your 10.5 inch if properly ported (and I suspect it is) has no shortage of gas pressure; in fact short barreled rifles often have a tendency toward being over gassed which can result in bolt bounce. Aside from bolt bounce (which can usually be cured by a heavy buffer too) additional gas also carries with it more carbon fouling.

These special breaks you mention probably don't raise pressure as much as they lengthen the pulse; which is good. Think of it as a wave, we want a wavelength not amplitude. But it probably isn't cheap and hard would require tools to change.

Try some other buffers first; it is cheap and required about 1 minute and no tools to switch. If a different buffer doesn't fix it then we can explore other options.
 

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Are you still having problems? I have several SBR's down to 6.5in and have experienced this problem before. What brand of upper did you get?
 

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Discussion Starter · #14 ·
utahEBM said:
Are you still having problems? I have several SBR's down to 6.5in and have experienced this problem before. What brand of upper did you get?
Since my original post I have probably only run about 200 rounds through my SBR. No problems. I haven't put the new buffer in yet. I keep waiting for a reason to order more than just the buffer. And it happens so infrequently I just haven't been in a rush to solve the problem plus summers are so darn busy for me I hardly even have time to think. I am confident the new buffer will solve the problem though. I have researched a bit more on some other forums and consistently found the same recommendation.

My upper is Rock River Arms.
 

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I've got an SBR with a 10" barrel, I put a Gemtech suppressor on the end of that, and my gun runs like a champ with regular cleaning, no complaints. Maybe try a suppressor, I hate shooting it with the short barrel without the suppressor, it is just too dang loud! Just my .02 cents. :wink:
 

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It sounds like the buffer to me. The bolt runs a lot faster on the shorter barrel guns, so much faster that the magazine cannot present the next round before the bolt starts closing. I have a Commando style 11.5" upper and with the standard buffer it runs OK but sometimes the mag catch does not hold the bolt open on an empty mag.

This thread may help:
http://www.m4carbine.net/showthread.php?t=873
 
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