AR SBR Build

**marstev** · September 15th, 2012, 09:05 PM

I have recently completed my first AR build, and made an SBR. It has a CMMG lower with separately purchased CMMG trigger group, and a Bushmaster 10.5" barrel. The rest of the parts are a mix of other generic parts from unknown manufacturers. The Bolt and Carrier group was the last item purchased at the last Oaks gunshow, government contract, DPMS I think, but not sure so I will assume generic government contract.

When I took it to the range, I experienced what I have read to be a typical feed problem with the first shot going every time, but the second round not loading, sometimes jamming, sometimes not ejecting the first. On first look, It appears that the carrier is not going all the way back.

One of the simple fixes I have read to try is to use a heavy buffer, or possibly an H2. My question, before buying more parts, is there any way to tell if I already have a heavy buffer installed? The one I have came from a lot purchase I made a while ago, that is why I do not really know what I have in there.

**animalmother85** · September 16th, 2012, 12:41 AM

It's usually stamped 'h' for a heavy buffer. If not, weigh it. You did do a form 1, right? Also, measure the gas port in the barrel. If it's a bushmaster, there's a good chance that it's undersized.

**hooknladder** · September 16th, 2012, 12:44 AM

The only problem I had with my 11.5" SBR was lack of extractor tension. Take your extractor out of the bolt and go to the hardware store. Buy a couple o-rings that just fit around the extractor spring. Reassemble. It may be tougher to get the pin back in. I believe mine only needed one o-ring.

Other than that, make sure your gas block is on straight. Mine was letting the bolt go back too fast, pulling the extractor over the case rim and feeding the next round into the fired case. It has been fine since I installed the o-ring years ago.

**marstev** · September 16th, 2012, 12:27 PM

Thanks for the responses...

Yes, I did do the Form 1 and have the stamp. I carry a photo copy with me to the range, for legal and bragging rights

Thank you, however, for asking. I know that is a common sense thing for me, but many others may not even be aware until it is too late.

The barrel I have came with the front sight installed, is there another way to check the gas port diameter?

I am taking it to the range today to try swapping my bolt/carrier group with another early AR, EA model J-15, to see if that may have something to do with it. The new bolt/carrier does seem to have a snugger fit than I am used to seeing in M16's. I just assumed it was due to higher wear in the 16's and mine might just need to be broken it a bit.

When I break it down today, I will look for the "H" mark on the buffer. I have also read on other boards about "tuned" springs. I am quite certain I do not have one of those, so that is another option I will be considering.

Also, the extractor tips sounded good as well. I don't think that is my issue yet, though it will probably come up as soon as I fix this one so I will be looking for that.

**animalmother85** · September 16th, 2012, 08:18 PM

Originally Posted by marstev

Thanks for the responses...

Yes, I did do the Form 1 and have the stamp. I carry a photo copy with me to the range, for legal and bragging rights

Thank you, however, for asking. I know that is a common sense thing for me, but many others may not even be aware until it is too late.

The barrel I have came with the front sight installed, is there another way to check the gas port diameter?

I am taking it to the range today to try swapping my bolt/carrier group with another early AR, EA model J-15, to see if that may have something to do with it. The new bolt/carrier does seem to have a snugger fit than I am used to seeing in M16's. I just assumed it was due to higher wear in the 16's and mine might just need to be broken it a bit.

When I break it down today, I will look for the "H" mark on the buffer. I have also read on other boards about "tuned" springs. I am quite certain I do not have one of those, so that is another option I will be considering.

Also, the extractor tips sounded good as well. I don't think that is my issue yet, though it will probably come up as soon as I fix this one so I will be looking for that.

I figured that you probably did being that you did post it in the correct sub-forum, but I just wanted to make sure. I'd hate to see a fellow gun owner get hemmed up over some stupid law about barrel length.

I don't 'tune' springs. I am guessing that they mean cutting the spring to a specific length. I don't really like the idea of doing it. I like to get the right spring for the job. That's not to the method that you are talking about it wrong, I just don't necessarily recommend it.

In order to check the gas port size, you will need to remove the gas block. You should use gage pins, but being that they are expensive for trivial usage. Try swinging by a machine shop to see if they have a set of gage pins.

Did you check the headspace?