So i had a chance to take my AR to the range and sight it in yesterday. I put about 200 rounds through it, performed great. After it got a little dirty (pleannnnttyyyy of clp in there) I noticed that the bolt was not staying back on the last round. Didn't bother me too much and still doesn't, but would like to get to the bottom of it. I was using the federal xm155 (not sure if that's the right number, but it is xm something) 55 gr ammo. I also loaded up a few 63 grain Wolf loads. It held back fine using those.


My thoughts are as follows:

The bcg is a ym nm. So it is a half oz heavier than mil spec. I am using the buffer spring from my Vltor Emod kit. I believe it is a carbine buffer spring/tube. My system is a mid-length.

My guess is the lower grain ammo is not powerful enough to kit the bcg back far enough to catch on the magazine. I was also thinking that maybe the spring is too stiff since it is meant for a carbine length system?

If I wanted to solve this would I need a softer spring?


Guessing on all of this, looking for your help!

Thx!

Were you using the same magazine, or did you try others? I have an old Colt magazine with a tilted follower that will not lock the bolt back after the last round. That could very well be your problem.

Your setup sounds fine. The XM193 should function right in your rifle.
What kind of magazines were you using?

I was using pmags. Tried a few different ones. Same thing happened.

Is it a factory rifle or something you put together?

Was put together through rainier arms.

Another issue could be the use of a disconnector spring in place of the normal bolt catch spring. I made this mistake on my A4, and the thing always failed to lock the bolt back after the last round. I replaced the disconnector spring with the proper spring, and I haven't had a problem since.

I would give it a good cleaning, lube it up and try it again. If you are still having problems I would contact Rainier arms for a solution.

Ok guys sounds good. Any more suggestions from others welcomed as well!

Thanks for the replies....

I would do the same, clean it and report back to rainer with any problems. Besides that how did she do?

Performed flawlessly besides that........

Someone on another forum thinks that my set-up is "heavy" and a C buffer would fix it in his mind?


Thoughts?

I thought you mentioned that you were using a carbine buffer. Based on your description, it actually sounds like yourrifle is overgassed quite a bit. The bolt might be cycling too fast because you mentioned that itwas malfunctioning with XM193 (hot loads) and working with Wolf (known to be underpowered). Try aheavier buffer like a H2 or 9mm.

Ahh ok C stands for carbine didn't catch onto that. I will mention that in the other thread. thanks.

I had the same problem a couple of months ago. I tried many different magazines , some would hold the bolt back others wouldn't.
All magazines were 20 rounders VietNam vintage. The mags that wouldn't hold the bolt back had new springs added. But one must realize that you could be in a situation where you don't have your personal mags with you and besides it gets expensive to replace all those mag springs, and besides it's a PIA.

I have several AR's and all the mags used in these rifles prior to the magazine spring replacements did hold the bolt back on the last shot. I investgated further to find that the rifle used where the bolt would not stay to the rear had a rather strong bolt catch spring, one could tell the difference between this bolt catch comparred to the rifles that didn't have that problem. I ordered a new spring from a different source and replaced the old one, problem solved. Every mag I've used locked the bolt to the rear on the last shot.

BTW, I did have proper recoil spring and buffer, the problem was a too strong bolt catch spring that prevented the magazine follower from activating the bolt catch.

Does it lock back consistently when you cycle manually? If so, then look at gas pressure (too little or too much), or try a different buffer. If it won't lock back solidly when you pull the charging handle and release on an empty mag then it's the mag or the bolt catch/spring (or, least likely, an out of spec lower).

I tried it on an empty mag and it locks back constantly. Right now I am using a C buffer so maybe use an H? My Bcg is staked fyi, how would a go about checking for leaks?

It sounds like you don't have cycling problems with a range of ammunition, so I would try a heavier buffer (guessing the gun is cycling too fast). Better, try a different upper with the same BCG. If everything works well you can rule out the lower and that leaves you with an upper with too much gas. And as someone mentioned, check that bolt catch spring (if it's the problem you should see it for any upper).

So what buffer would you buy? H?

Yes, but I would try a different upper first if possible.

I worked on a DPMS parts gun with similar problems. It ended up being a loose gas key. Try another known bolt carrier to rule out a leaking gas key.