Sig Sauer M400 Enhanced AR-15 - Bolt Not Locking Back After Last Round Problem

Raff85

Raff85

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Hi All,

I have a Sig Sauer M400 Enhanced, I have put about 750 rounds through it and just this last weekend I was out at the range and noticed that after that after last round was fired the bolt would seldom lock back.

I have tried two different types of ammo, and the both types of ammo were also used in my co-workers Norinco AR which has no problems. The ammo used were 62 GR 5.56 LAP by PMC and 55 GR .223 by Federal.

I also tried several different mags, including Pmags, USGI, and IMI Defense.

I didn't notice any gas leaks while firing. The bolt catch seems to be working fine and when manually racked with an empty mag it the bolt locks back as it should.

Parts are lubed, but not to excess.
 
Try this:

Insert an empty mag into the gun, and pull the bottom of the mag to the rear, so that the top of the mag is closer top the barrel. While doing this, pull the charging handle back and see if the follower on the mag slips past the bolt stop.
 
This is a tough one ... I've got the same rifle and it's pretty much perfect...maybe mags are not seating all the way? I find some of the rivets make 5 rounds pretty tight and you really have to work it in to get it to lock the mag in...or the bolt catch has had the top lip busted off...
 
I can think of a few things that may cause this:

- Mag catch location milled into the lower receiver is on the low tolerance side but high enough for the bolt to strip a round but not high enough to push the bolt catch to make a solid lockup.

- Worn or slightly smaller profile on bolt catch just not high enough to make a solid lockup on the bolt.

- Rifle undergassed due to misaligned or too small a barrel gas port. I doubt this is the case as you might have experienced the odd short stroking.

- Rifle over gassed and buffer weight too light, not giving enough time for the magazine's spring to push up the empty follower in time to engage the bolt catch and arrest the bolt. Got an H2 or H3 buffer kicking around to test?
 
Still working on it, been a busy few days at work. Going to try some suggestions tonight. Would it help if I posted pictures as well?
 
Trinimon said:
I can think of a few things that may cause this:

- Mag catch location milled into the lower receiver is on the low tolerance side but high enough for the bolt to strip a round but not high enough to push the bolt catch to make a solid lockup.

- Worn or slightly smaller profile on bolt catch just not high enough to make a solid lockup on the bolt.

- Rifle undergassed due to misaligned or too small a barrel gas port. I doubt this is the case as you might have experienced the odd short stroking.

- Rifle over gassed and buffer weight too light, not giving enough time for the magazine's spring to push up the empty follower in time to engage the bolt catch and arrest the bolt. Got an H2 or H3 buffer kicking around to test?
Click to expand...


Any way to test some of these? I don't have any other spare parts laying around...
 
To the OP. This sounds like a magazine related issue. A lot of pinned rifle magazines are pinned too shallow and will end up not allowing the follower to relax to its full position which ultimately I'll not induce proper bolt lock.
 
Raff85 said:
Any way to test some of these? I don't have any other spare parts laying around...
Click to expand...

Got to the range with your buddy next time and swap uppers and see if the problem occurs again. Swap buffer springs/buffer see if the problem persists. Swap BCGs and see if it still happens.

Non-phixion said:
To the OP. This sounds like a magazine related issue. A lot of pinned rifle magazines are pinned too shallow and will end up not allowing the follower to relax to its full position which ultimately I'll not induce proper bolt lock.
Click to expand...

Doesn't really sound like a mag issue to me seeing that the OP tried 3 different brands and all had the same issue.
 
You're not unconsciously riding the paddle with your support hand while shooting are you?
Sounds far fetched but ruling out possibilities! If you hold your rifle by the magwell, you could unconsciously press the paddle while shooting esp if you have long thumbs!
 
Everything looks fine in these piks. Only thing I can think of is the mag isn't seated fully. How many rounds through the gun?
Never mind. just read the first post again.
Odd issue no doubt
 
Yeah only about 750 or so.

Like I said this weekend I'll try and switch out some parts with my buddies Norinco AR and see if I can localize the problem. If not then I will take some slow-mo videos to post on here.

Kinda sucks that I'm located in the middle of no where and no gunsmiths are around...
 
If you pull the buffer/spring out...nothing in there?
Strip the bolt and run a pipe cleaner through it? Maybe some gunk inside the gas tube is impeding it by just a hair.
Just random thoughts
 
Raff85 said:
Yeah only about 750 or so.

Like I said this weekend I'll try and switch out some parts with my buddies Norinco AR and see if I can localize the problem. If not then I will take some slow-mo videos to post on here.

Kinda sucks that I'm located in the middle of no where and no gunsmiths are around...
Click to expand...
Don't swap parts from a norc into your sig. Most norc parts are Chinese spec.
 
bsand said:
Don't swap parts from a norc into your sig. Most norc parts are Chinese spec.
Click to expand...

Folks have interchanged uppers/lowers and other Norc parts many times from buffer tubes, buffer weights, trigger groups and upper/lower receivers. Heck, someone swapped out just the barrel of a Norc and shot sub-moa groups. Stop spreading mis-information.
 
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