Bolt locking issues

товарищ · #1 04-06-12, 22:01

On the last round of every mag, the bolt is not catching. I'm not using a BAD lever or anything. This occurs 100% of the time.

The lower that I'm using is a Mega forged lower with Grant's LPK (LMT parts, right?) and a BCM carbine RE/spring with an H2 buffer. The upper that it's being used with has been tested with two other lowers without having any issues. Upper is a 16" mid-length, made from BCM/DD parts, and the gas system was checked out by ADCO. So far I've sifted through a few things that could cause problems:

1. It's not the mags, I went through 8 different mags/types.
2. System isn't over/undergassed, ejection is fine (ejects 3 o'clock).
3. Isn't ammunition (XM193)
4. Isn't the BCG, gas key and rings are fine.

The upper ran fine with another lower on a Vltor A5 system. So I've narrowed it down to this:

1. Buffer is too heavy (H/H2 were tried, haven't tried a standard)
2. Bolt catch, spring, or plunger is defective (Bolt catch will hold/release fine when pressed/depressed)
3. Out of spec lower

Are there any other possibilities for problems? I'm not sure if it's a buffer weight issue because I've ran this upper with an A5, which is heavier. One thing I'm noticing is that the bolt catch sticks out halfway when it fails to catch the bolt.

djmorris · #2 04-06-12, 23:29

Who assembled the LPK and also have you inspected the bolt catch and spring closely?

Doesn't sound like it's at all related to a gas/buffer issue, at least not to me. That being said, any particular reason for running an H2 in the 16" middy? I'd start by replacing it with a standard or H buffer - then start gettin' down to the nitty gritty and only after identifying and fixing the initial issue would I throw my H2 back in.

товарищ · #3 04-06-12, 23:33

Quote:

Originally Posted by djmorris

Doesn't sound like it's at all related to a gas/buffer issue, at least not to me.

Who assembled the LPK and also have you inspected the bolt catch and spring closely?

Any particular reason for running an H2 in the 16" middy? Foremost, I'd start by replacing it with a standard or H buffer - then start gettin' down to the nitty gritty.

I assembled the lower (sixth time I've done it, never had an issue before) and ensured that it was done properly. The only thing different this time was the LPK. The bolt catch, spring, and plunger move properly and they look fine.

I've used H/H2 buffers before on comparable rifles. I know for sure that it shouldn't have any issues with an H buffer, but I found a standard buffer that I'll try out tomorrow.

djmorris · #4 04-06-12, 23:43

Right. Sorry I did a quick edit for a little bit of clarification. I'm not implying the H2 would cause issues but if you're trying to weed an issue out it's probably best to stay as close to a 'standard' configuration as possible.

Another possibility, although probably a long shot, have you run any ammo from a different batch through it yet? I'm assuming this is a newly assembled build, not alot of rounds through it..?

товарищ · #5 04-06-12, 23:47

Quote:

Originally Posted by djmorris

Right. Sorry I did a quick edit for a little bit of clarification. I'm not implying the H2 would cause issues but if you're trying to weed an issue out it's probably best to stay as close to a 'standard' configuration as possible.

Another possibility, although probably a long shot, have you run any ammo from a different batch through it yet? I'm assuming this is a newly assembled build, not alot of rounds through it..?

It's a valid concern because a lot of people don't bother with H2 and heavier buffers for this type of setup.

The lower is new, but the upper has about 1k through it. The XM193 used was from two different lots.

Iraqgunz · #6 04-07-12, 07:05

Please remove that BAD lever and then report back to us. Do some searching and you will find similar issues.

5pins · #7 04-07-12, 08:14

He said he was not using a BAD.

Iraqgunz · #8 04-07-12, 08:42

I misread. I would replace the buffer with a standard carbine buffer.

Quote:

Originally Posted by 5pins

He said he was not using a BAD.

Robb Jensen · #9 04-07-12, 08:46

H buffer is the heaviest I use on a mid-length. H2s are for CAR gas guns and/or suppressed guns.

chamber143 · #10 04-07-12, 22:26

I use a h2 in 16" bcm middy. Doesn't mean anything though, try replacing the bolt catch

GunFighter98 · #11 04-07-12, 23:38

+ 1 on checking the bolt catch.

trlcavscout · #12 04-07-12, 23:45

Most of these I have seen the spring was mixed up with the disconnector spring or was installed wrong. Do you have another spring to try?

товарищ · #13 04-08-12, 13:39

Quote:

Originally Posted by Robb Jensen

H buffer is the heaviest I use on a mid-length. H2s are for CAR gas guns and/or suppressed guns.

I'm going to order a standard today and see how it works later this week. Thanks.

Quote:

Originally Posted by trlcavscout

Most of these I have seen the spring was mixed up with the disconnector spring or was installed wrong. Do you have another spring to try?

Nope, I keep my FCG parts away from everything else. I have a separate lower with a DD LPK and carbine RE, so I'm going to throw the H2 into that and try it with my upper. I wish I would have remembered to bring it a few days ago.

Todd00000 · #14 04-09-12, 08:07

I had the nipple on the bolt catch, the protrusion that the mag follower comes in contact with, break off. Check your bolt catch with your upper off and an empty mag in the well. It's easy to miss.

ASH556 · #15 04-09-12, 17:18

My money's on short stroking/H2 buffer. Run a std H buffer and see what happens.

jstyer · #16 04-09-12, 21:34

Quote:

Originally Posted by ASH556

My money's on short stroking/H2 buffer. Run a std H buffer and see what happens.

+1 on this... I'd definitely run an H buffer before diagnosing anything else. It's a cheep/quick check.

товарищ · #17 04-13-12, 17:33

I got around to shooting again today and now I have a new problem.

I tried a BCM standard & H buffer. Both buffers will lock back only 90% of the time.

Internals are fine, nothing is broken or defective. Ammo used was from a different lot of Federal M193. I ran 30 rounds of 75gr .223 Hornady TAP and the bolt locked back for all of the rounds.

I've never had a problem with Federal M193 being underpowered (and I don't think the ammo is at fault here), so this is kind of weird.

Rifleman_04 · #18 04-13-12, 18:11

I recently replaced a slightly out of spec LMT bolt catch. Might wanna give that a try if you have a spare laying around.

GunFighter98 · #19 04-13-12, 19:28

Quote:

Originally Posted by Rifleman_04

I recently replaced a slightly out of spec LMT bolt catch. Might wanna give that a try if you have a spare laying around.

Thanks, works flawless now!

товарищ · #20 04-13-12, 19:44

Quote:

Originally Posted by Rifleman_04

I recently replaced a slightly out of spec LMT bolt catch. Might wanna give that a try if you have a spare laying around.

I don't have a spare, but I'm going to retry this tomorrow with a DD catch. I should have just stuck with a DD LPK.