Bolt not staying back after last shot...

[Deadduck]

Hey all,
A friend and I went shooting today and I tried out a new upper, an Armalite ISP Carbine M4 upper. Fed and fired fine, good accuracy, but the bolt wouldn't catch after the last round in the mag. Used several different GI mags by Colt and Parsons, and some CProducts mags, so I don't think its a mag problem. Couldn't tell that it was short stroking or anything, and it was feeding properly.

Any ideas of what I need to look for? :confused:

Thanks,
Keith

[SHIVAN]

Ammo?

[M4arc]

It sounds like it's short stroking. Like Shivan asked, what ammo were you running?

[Deadduck]

55 Grain Remington FMJ. Didn't try any M855.

[M4arc]

55 Grain Remington FMJ. Didn't try any M855.

The stuff in the green & white box that they sell at Wal-Mart?

[Deadduck]

Yep. Has run fine in other rifles. Doesn't seem too weak

[M4arc]

It is a light load though so that might be your problem. Try this; take some of that and some M855. Load one round in the magazine and fire said round. See if the bolt locks back. Try this with both the Remington and M855 and see if it's an issue with both. I'm pretty sure you'll see this only with the Remington.

[BushmasterFanBoy]

FWIW, both of my AR's run this ammo fine. First I'd test it against M855, then I'd look at something rifle related.

[Deadduck]

Ok. I'll try it tomorrow if I get a chance and post back. This ammo has run fine with this lower with my old upper. Do you think there's a gas system problem, or might it just need to get broken in? It was well lubed.

[CM-4]

Next time you shoot, measure how far away the brass is landing. The brass should at least be landing a foot away from where you are standing, most of the time more than that. If it is than short stroking may not be the culprit. The first thing I would look at is your mags again. How old are these mags? How old are the springs? The reason I say this is because I had an issue with an 11.5" upper that I thought was SS.

Like yours it would not hold the bolt back on the last round. I tried several different mags and on all mags it did the same thing. However, when I would try the same mags on a different rifle, the bolt would hold open just fine. I talked to DANO who is also on this sight and might weigh in too, he asked me to measure my brass. My rifle was spitting the brass out about 3 feet from me so he said that it probably wasn't SS.

So I checked my bolt and carrier and made sure everything was tight and not leaking gas. Everything was fine. So I seperated the upper from the lower and put a mag in the well so I could examine the bolt catch. I noticed that when I would push the mag follower down and let it snap up, the catch was only being lifted about half of the way up and I could take my finger and push it the rest of the way up. On the other rifle the catch must have been broke in a little more because it would go up about 3/4 of the way up, just enough to catch the bolt.

The next day I was with a friend who had just bought a new 20 round mag. I put that in the mag well and noticed that the catch went all of the way up. I loaded 50 rounds one at a time in that mag and on every one the bolt locked back, so I ordered new springs for all of my mags and haven't had a problem since.

You might also check you bolt catch and make sure that it is not binding or weak. I would do the same thing and just put an empty mag in the well and push the follower down and make sure it is not binding. If you mags are fine and the catch is fine then I would look at your buffer and buffer spring. Hope this helps and sorry for the long winded reply.

[Deadduck]

Brass is going 4-5 feet, about the same as the other rifle we were shooting with the same ammo. CProducts mags are new with Magpul followers, and I know that I just put a new spring & green follower in one of the USGI mags, others are old.
I'll look at the way the mag interacts with the bolt stop tomorrow.
Thanks,
Keith

[SHIVAN]

Is the gas block their 2 set screw version? If so, look and see if it's canted at all, you may be choking some of the gas off, but I doubt it.

Second, check the carrier key for both staking and loose screws. It may not be staked thoroughly and it could be loose. I doubt it, but you should check.

If it is loose, back the screws out and re-tighten. Then stake it.

I'm betting the ammo is a little light and is just missing bolt stop. Especially if you are ejecting 4-5' and there is no spent brass left in the breech or chamber.

[Remember911]

I have had some trouble with this in my 10.3 I have found that with a light load and heavier buffer spring and buffer (that I like to run in it) the bolt at times does not stay back. I've learned to just shoot the good stuff,and use the other stuff in my guns with a lighter buffer spring and buffer.

[Iraqgunz]

I had a Junkmaster here in the armory that was doing the same thing. I changed the bolt lock/ release as well as the spring and it has been fine since then. I am pretty sure that there is no short stroking/ gas related issues here. Unfortunately not seeing it in person is like giving medical advice over the phone.

[Remember911]

I had a Junkmaster here in the armory that was doing the same thing. I changed the bolt lock/ release as well as the spring and it has been fine since then. I am pretty sure that there is no short stroking/ gas related issues here. Unfortunately not seeing it in person is like giving medical advice over the phone.
This I believe to be very true. So many things that it could be.

[Deadduck]

Finally was able to check out my bolt catch problem, and luckily I don't think it's a short stroking problem.

There was a tiny burr on the back of the bolt catch that only binds when the follower engages the catch. Worked fine when manually engaged from the outside, but try and move it from the front when it contacts the mag follower, and it will sometimes bind. I put the lower together myself and should have caught that. Have it smoothed out now, and I'll shoot it tomorrow, but it seems fine now. Works like it should.

So, it's my self assembled lower and not the Armalite upper that caused the problem. The upper ran 100%.

Thanks for your replies!