Premature lockback?

[Amicus]

I've seen this with pistols, but rarely with M4-ish carbines. Does anyone have an idea why this happened?

Yesterday I had lockback with one cartridge left in the mag. Particulars:

Cartridge: LC M855

Mag:

English steel military mag (black)
Magpul Gen 3 follower
Original spring

Upper:

DD barrel and Vltor upper, assembled by DD (long story)

I know someone is going to ding me for the mag, but I have had good results from them, generally.

[Onyx Z]

I'm putting my money on the mag. What LPK? Could be the bolt catch is out of spec also.

[dramabeats]

What kind of bolt? if it's high quality then that mag is probably causing your issue.

[Amicus]

What kind of bolt? if it's high quality then that mag is probably causing your issue.

BCM. Buffer system is Vltor A5.

Probably the mag, huh? Perhaps the mag design is allowing the Magpul follower to protrude too high?

[Iraqgunz]

So let me get this straight.

You ask what could cause it and then state you might get dinged for the magazine that you "generally have had good results from". So I have to ask, why bother?

It's also a good idea to know who made the lower. Magazines and ammo are the top causes of failures in addition to improperly assembled AR's which is occurring more and more.

If you want to find the problem, give us all the details. Then try a magazine that is KNOWN to function.

[Amicus]

So let me get this straight.

You ask what could cause it and then state you might get dinged for the magazine that you "generally have had good results from". So I have to ask, why bother?

It's also a good idea to know who made the lower. Magazines and ammo are the top causes of failures in addition to improperly assembled AR's which is occurring more and more.

If you want to find the problem, give us all the details. Then try a magazine that is KNOWN to function.

IG:

The only reason I blamed the magazine is that I assumed that was the culprit, but I am not sure. I have seen this with pistols, but not with the AR type rifles. I thought it was exceptionally peculiar because I thought that the cartridge would prevent the bolt release from functioning to lock back the BCG. To put it another way, I thought that the cartridge would have depressed the follower enough into the magazine body to prevent the follower from activating the bolt catch/release.

The lower is a Colt (preban, one of my "behind enemy lines" carbines), with a Vltor A5 (RE, buffer, and Springco rifle spring) system and a Geissele SSA trigger. Both installed by me. In essence, it is an entirely rebuilt carbine.

IIRC, this particular carbine has never malfed, but it has fewer than 1000 rounds through it since I acquired it (probably closer to 500). During this session, I used three USGI mags and three English steel mags, each initially loaded with 28 cartridges. All ammo was LC M855. This was the only "irregularity."

This is the best scenario I have come up with:

I thought that the follower had to be close to to the top of the magazine box to cause a lock back. With a cartridge remaining, that could not be the case. I thought that it was possible for the cartridge to have slipped forward slightly, possibly due to the bolt riding over the cartridge, allowing the back end of the follower to tilt upward, activating the bolt lock. So, when I removed the box (not realizing at the time that there was a malf) the tip of the bullet was pushed backward by contact with the front of the inside of the magwell, and the cartridge was pushed back into place. But, with Magpul anti-tilt followers, that seemed unlikely.

Perhaps the English (Imperial Defense) springs in the mags are weak, allowing the bolt to override and starting this mess. I bought them "new in wrap" some years ago, but have just started using them. I just want to know if anyone has any ideas so I may repair this.

[Iraqgunz]

First thing I would do is use some good magazines and shoot it again. Something tells me it is the problem. I could be completely wrong.

[elgie]

I'd check for a missing plunger or weak spring under the bolt catch/release as well. Could be weak spring tension letting it rattle around..

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[Amicus]

I'd check for a missing plunger or weak spring under the bolt catch/release as well. Could be weak spring tension letting it rattle around..

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Yeah, that's a good idea too. You never know about some of these carbines with 25 year old spring sets. This one had a very low round count, measured by wear when I bought it, but crap happens. Some springs are no good from the get go. (Had that happen with a bolt lock spring before.)

A few years ago, I had a preban for three days, cleaned it (but did not disassemble the bolt), and took it to a class. There's a 50/50 chance it was the same carbine lower. I lent it to a student. After a few rounds he had a bad doublefeed. Clearing the case required bench work; it just wouldn't mortar out.

When I pulled the bolt and checked it, I discovered it had the old Colt copper-colored extractor spring with the blue insert. Apparently, that insert did not hold up over time. Replacement of the insert and spring made the carbine reliable again. A $1000 carbine brought low by $2 worth of parts that any decent armorer would have replaced years ago.

[Dead Man]

If it happens again, look inside the ejection port and see if you can see why it's locked back. All of the parts involved are visible, afterall.

I'm having trouble visualizing how a magazine can lock a weapon back when the follower is still under a cartridge, myself. I suppose a really shot magazine can behave strangely; allowing the cartridge rim to actuate the bolt stop? Maybe the spine of the magazine is somehow riding up high enough to hit it? Really seems like a bad spring is more likely, to me. But it's only happened once; "freak" things do happen sometiems.