Newbie Question - Is this short stroking?

Ok I apologize in advance, Im new and just learning with these rifles. I had a friend of mine that builds AR15s set one up for me a couple of months ago.
As far as I know, it has:

Spike's Upper
Green Mountain Barrel
Stag Arms Lower
"Stock" Buffer Assembly
Came with a Standard BCG, I swapped out to a BCM
Low-Profile Gas Block

So, the problem:



Partway through a mag I will get this. From the research I have done on here it looks like short stroking. BUT I only get this one every say 3 mags. Not consistant as to which mag, I have 5 pmags and 1 USGI and I think it has only happened on the pmags, but odds are it would anyways. It locks back after the mag is empty every time, which i do't think would happen if you have a short stroking problem.

I have had it happen with .223 PMC bronze and XM193. I though it might just be break in, which was the PMC. After 500 rounds of that I switched to XM193, and I have put ~200 rounds of that through. It did it twice with the XM193 in those 200 rounds. It has also done it with both BCGs.

Any suggestions? It seems like most problems with short stroking are much more consistant and severe than what I have going on, which is why I'm not sure if that is what it is. I will be talking to the builder tonight (works nights, hard to get a hold of) but I want to keep myself educated and find out if its anything I can fix myself.

Thanks for any and all help.

If your bolt is locking back it is not short stroking. It could be over gassed though and the bolt is returning forward so fast that the round does not have time to be lifted from the mag and picked up by the bolt. I had this happen on my .500 Phantom and it drove me nuts trying to figure it out. The solution in your case might be as simple as using an extra power recoil spring or if that does not do it, add a HB to slow things down. If both do not slow it down enough and it is still locking back on every empty mag, then an adjustable gas block will do the trick. If you go spring, buffer, gas block you are going from cheapest to most expensive. I like to go cheap first and work my way up to the more expensive and more complicated to replace parts.

Also, where are the ejected rounds landing? If they are being ejected in the 12:00-2:00 position then you are more than likely over gassed. If the rifle were truly short stroking then the bolt would not be locking back and ejection would likely be in the 4:00 to 6:00 range.

Of course, make sure the rounds are moving freely in the magazine. If they are a bit too long and binding in the mag then there's your problem. Clean and lightly lube the interior of the mag and make sure the rounds have room to rise as the follower pushes them up and that the follower is not binding. This would be the first thing I'd check.

Bolt locking back after the last shot, it full stroking, and it all the mag at fault.

The problem is that the bolt on the way forward is not getting behind the rim of the round, but grabbing the round via the middle of the case in front of the round to try to strip the round out of the mag that way instead (read jam is with the bolt on top of the middle of the case, the tip of the round in the feed ramps, and bolt bottom lugs doing the damage to the case.


So on that note, knowing that it a problem with the mag somehow, lets focus on that.

Start off by fully stripping all your mags down to CLP clean them. Even new mags can have shop debries in them, and can cause problems with the mag not fully recovering in time before the makes it way back forward to correctly strip the round (read as the bolt is locked home, it has the top round in the mag compressed down, and as the bolt come back, the top round has to fully seat against the feed lips in order for the bolt to be able to strip it correctly out of the mag.

Next, make sure that the threaded section shaft end of the mag catch is Flush with the face of the mag release button. To adjust the catch if needed, push the mag release button all the way in with the end of a pencil, then you can turn the catch side since it will now be protruding out the other side of the receiver. If you find that you are either a 1/2 wind in or out of the threads being flush with the button, pull the button, turn it 180 in the channel, then reinstall the mag catch.

From there, with a mag inserted in the mag well, how much slop do you have with being able to move the bottom of the mag forward and aft. Also, If you are using the mag as a hand rest/grab point, or using it as a rest on the bench, it possible to cam the mag in the well rear top back downwards to induce this type of jam of the bolt missing the rim on the way forward stoke.

Lastly, with the catch out of the rifle, and the bolt locked home, shove a mag body (without follower or spring) all the way into the well as far as it will go up, and compare the mag's catch slots to the receiver catch channel. You should be able to shove the mag up about a 1/16" more that if it was locked in by the catch, but if you find that you can shove the mag up much father than this until the mag lips bottom out on the bottom of the carrier reliefs, then you may have a lower receiver out of spec.

Both sound like possible causes of my problem. Let me try to provide more info to see if it will help eliminate anything. I don't have my hands on my rifle at the moment, I just dropped off at the gunsmith. Long story short, the guy that assembeled it must have used red loctite on the set screw that holds the free float handguard on. Its an aluminum set screw and stripped as soon as I put pressure on the wrench. Since I don't trust something I can't take apart I brought it to a shop, and they are going to check things out a bit while it is there.

So if it's the recoil spring, would it be that it is too strong, and returning the bolt forward too soon? I also wonder if the low profile gas block could be letting too much gas back? The spent brass typically lands in the 3:00 to 4:00 area I think, I haven't paid too much attention to it but I will next time I shoot.

As far as mags go, when I was shooting the PMC I only had 3 mags, and it didn't seem to matter which one I used. Now when I switched to XM193 I also picked up 3 more pmags. The 2 failures from shooting last time happened on these new pmags. Could there be an issue with this rifle not liking pmags? I left 2 of them with the gunsmith but I will take the others down and make sure they are clean.

I will check mag depth and the other things you suggested once I get the rifle back, but I will note that I don't shoot gripping the magwell, I have a vertical grip. So it shouldn't be being pulled back at all.

It sounds like either the bolt is coming forward too soon, or the mag isn't pushing the round far enough up, fast enough. I can grasp both of those scenarios.
Thanks for all the help so far, I hate not knowing how something like this should run.

Normal for the gas block allen screws to be loctite on. Also on the float tube screws if its the kind I am thinking about too.

In both cases, you use a donor allen wench in the allen sockets, then heat up the allen wrench to transfer 600* heat to the screws to break the loctite bond; then change out the donor wrench to a working one and remove/loosen the bolts.

Again, if the bolt is coming back so the catch can lock it back after the last round fired from the mag, stroke is fine,and it's either the mag not recovering correctly or quick enough before the bolt makes it forward stroke.

Granted that if the buffer in play is too light, you can loose the rear of stall dead blow effect of the B/C at the back of stroke (allows a little more time for the mag to recover before the bolt starts charging forward), but having Mac Jacked a few full auto rigs (no rear dead blow stall, and cycling at over 1200rps), unless the follower is really binding inside the mag, and the reason that the mag is not recovering quick enough, should not be the problem at hand if the mag body is in the right position to start with.

Would dare to guess as stated, mags needed to be cleaned, or just the mag not being correctly retained high enough in the lower receiver well for the back of the rim top of the stack to stay high enough that the bolt can not get over the top of the rim to grab the middle of the round instead.

It was the float tube screw, a light aluminum one so it stripped like nothing. Ah well, too late now.

Do mags need to be "broken in"? Maybe a couple of cycles of being used plus some clp will speed them up a bit.

I took the pmags I have at home and cycled them a bunch with a screwdriver. I took the spring out of a couple of them and sliding them in and out without spring tension you can feel them drag a tiny bit through the middle of the mag. I think they just need to be broken in, my oldest pmags had been cycled less than 5 times and my newest less than 2. And since I can't ever remember this happening with the USGI mag I have, I'm going to work all the mags and shoot again next week to see what happens.

Thanks for the help.

Copyright © 1996 - 2013 :: AR15.Com LLC :: All Rights Reserved.
Any use of this content without express written consent is prohibited.