I have a BCM upper with a full auto BCM BCG. It would not lock back on the last shot. I tried 4 different mags, and it would not lock back with any of them. However, it DID lock back when I manually charged the charging handle with an empty mag. Also, one time, it did lock back after the last shot, when I rapid fired 10 rounds. Otherwise, it just did not lock back. What can I do to troubleshoot the issue? Thanks in advance.

Do you have a B.A.D lever on it? I had one on my first AR and had the same problem until I removed it. Also what type of ammo? Do you have an adjustable gas block?

Do you have a B.A.D lever on it? I had one on my first AR and had the same problem until I removed it. Also what type of ammo? Do you have an adjustable gas block?

No BAD lever.

Ammo was 62 grain IMI M855 ammo, and 55 grain PMC ammo. I put the upper on my M&P lower. It's the first time I paired the two, and the first time I used the BCM BCG. I have a feeling it's not a BCG issue. No adjustable gas block.

What buffer do you have in? Sounds like short stroking. If heavy try a standard.

And what gas system?

 

What buffer do you have in? Sounds like short stroking. If heavy try a standard.

The buffer is the original one that came with the M&P15. I haven't had short stroking issues with it when paired with my M&P upper, but I agree that it looks to be short stroking.

 

And what gas system?

Carbine gas port.

Odd, is it well lubed?

Odd, is it well lubed?

It is. The odd thing is that the one time it did lock back was when I was rapid firing. Kind of makes me think it's short stroking. But if that's the case, wouldn't it do the same with the other upper as well?

Two variables on the lower are the bolt catch and the buffer weight. The upper may have a different gas port size it could be the gas block the bolt carrier key. May just need more lube.

 

Just to be clear it's a BCM carbine gas length upper with a carbine weight buffer

 

Two variables on the lower are the bolt catch and the buffer weight. The upper may have a different gas port size it could be the gas block the bolt carrier key. May just need more lube. Just to be clear it's a BCM carbine gas length upper with a carbine weight buffer

 

Hmm. tbh, it's the original buffer that came with the M&P, but I'm pretty sure it is a carbine weight buffer. This is the upper.

http://www.bravocompanyusa.com/BCM-16-BFH-Mid-Length-Upper-Receiver-Daniel-p/bcm-urg-mid-16%20bfh%20risii%20de.htm

Ok that's a mid length gas system and bcm's are notorious for having a small gas port size. I believe bcm recommends using a standard carbine buffer because of this. If you check the buffer does it I say H, or H1 or the like?

 

 

 

Nice upper btw

 

Ok that's a mid length gas system and bcm's are notorious for having a small gas port size. I believe bcm recommends using a standard carbine buffer because of this. If you check the buffer does it I say H, or H1 or the like?

I'm at work, but will check it out tonight when I get home.

 

Nice upper btw

Thank you. Also, thanks so much for your help!

Ok. No prob. Other than the buffer you can look at the gas key to make sure its nicely staked, but with bcm it usually is.

 

Ok that's a mid length gas system and bcm's are notorious for having a small gas port size. I believe bcm recommends using a standard carbine buffer because of this. If you check the buffer does it I say H, or H1 or the like?

Thank you!!! I've been shouting this from the rooftops for a couple years and people always shout me down and tell me there is nothing to be said about BCM.

 

Where did you hear this?

 

It's short stroking. BCMs are undergassed in my personal opinion. They say they are "optimized" for military ammo. It makes them a lot of fun to shoot, but if anything goes wrong I expect they will go down. Consider the buffer change (makes sense but I have no personal experience).

Thank you!!! I've been shouting this from the rooftops for a couple years and people always shout me down and tell me there is nothing to be said about BCM.

 

Where did you hear this?

 

It's short stroking. BCMs are undergassed in my personal opinion. They say they are "optimized" for military ammo. It makes them a lot of fun to shoot, but if anything goes wrong I expect they will go down. Consider the buffer change (makes sense but I have no personal experience).

 

It has that on certain mid length models. Right on their website.

 

I know I buy the cheapest brass I can find so carbine weight it is.

 

 

It has that on certain mid length models. Right on their website.

 

I know I buy the cheapest brass I can find so carbine weight it is.

 

So it's definitely short stroking. Is IMI M855 that underpowdered that it could cause it to short stroke? I understand it can and will with PMC, but I'm surprised about the IMI.

M855 should be gtg but I always use a standard buffer in my bcm middy cause I know it works.

M855 should be gtg but I always use a standard buffer in my bcm middy cause I know it works.

I'm going to check the buffer tonight and will report back, granted my 5 and a half month old son gives me 10 minutes. But it definitely sounds like that may be my issue.

Buffers can be checked with a scale.

 

Carbine - 2.9 oz

H1 - 3.8 oz

H2 - 4.6 oz

H3 - 5.6 oz

 

Check your spring also. Mil specs are:

Carbine 10 1/16" min to 11 1/4" max

Rifle       11 3/4" min to 13 1/2" max 

 

 

Careful, if that M&P lower was a ban state factory build with a factory pinned stock, it may have a rifle buffer in it.  Pinned factory stocks for banned states may look like a carbine stock, but the buffer system is rifle.  Not sure what S&W is using/doing. There are some that a putting a hybrid tube on the rifle, a rifle length tube with a carbine pinned stock, internally blocked to carbine length.  I suspect that is what you. Check pictures for carbine and rifle buffers before you go changing them.  Check your spring. You may have the wrong length.

 

Check your brass for extractor marks.  Check your extractor on the bolt. 

The BCG not locking back on the last round is a classic sign of being undergassed. Checko covered the BCM disclaimer.

 

I have H buffers in both of my BCM Midlength rifles, and they both run like a striped assed ape. But I only shoot mil-spec ammo or equivalent.

IMI is not weak, it has a good mil-spec reputation. Probably your gun barely slipped the BCM gas spec. Try the buffer. Don't leave it loaded with .223 for home defense, use 5.56.

 

You could also call BCM and tell them you want to swap the upper.

IMI is the Isreali Milsurp ammo right? Generally it is labeled as "XM" in this instance "XM855". The "X" denotes it was a rejected lot. I wouldn't trust that to trouble shoot a BCM. That just me though.

 

http://www.thegunzone.com/556xm.html

IMI is the Isreali Milsurp ammo right? Generally it is labeled as "XM" in this instance "XM855". The "X" denotes it was a rejected lot. I wouldn't trust that to trouble shoot a BCM. That just me though.

 

http://www.thegunzone.com/556xm.html

It's marked M855. No X.

Good deal. Disregard then.

IMI is not weak, it has a good mil-spec reputation. Probably your gun barely slipped the BCM gas spec. Try the buffer. Don't leave it loaded with .223 for home defense, use 5.56. You could also call BCM and tell them you want to swap the upper.

 

For sure I leave it loaded with 5.56.

Buffers can be checked with a scale.

 

Carbine - 2.9 oz

H1 - 3.8 oz

H2 - 4.6 oz

H3 - 5.6 oz

 

Check your spring also. Mil specs are:

Carbine 10 1/16" min to 11 1/4" max

Rifle       11 3/4" min to 13 1/2" max 

 

 

Careful, if that M&P lower was a ban state factory build with a factory pinned stock, it may have a rifle buffer in it.  Pinned factory stocks for banned states may look like a carbine stock, but the buffer system is rifle.  Not sure what S&W is using/doing. There are some that a putting a hybrid tube on the rifle, a rifle length tube with a carbine pinned stock, internally blocked to carbine length.  I suspect that is what you. Check pictures for carbine and rifle buffers before you go changing them.  Check your spring. You may have the wrong length.

 

Check your brass for extractor marks.  Check your extractor on the bolt. 

Thanks for the info!

Don't leave it loaded with .223 for home defense, use 5.56.

 

  

 

For sure I leave it loaded with 5.56.

What's wrong with .223?

 

I use Fed TRU 62gr Hi-Shok SP (http://le.atk.com/ammunition/federal/rifle/details.aspx?id=684) which is a .223 offering for all my guns set up for social purposes - including HD. It is also the standard issued round for many LE agencies and SWAT teams - mine included - and is regularly in the top choices of available rounds out there by the better known ballistics guys.

What's wrong with .223?

 

I use Fed TRU 62gr Hi-Shok SP (http://le.atk.com/ammunition/federal/rifle/details.aspx?id=684) which is a .223 offering for all my guns set up for social purposes - including HD. It is also the standard issued round for many LE agencies and SWAT teams - mine included - and is regularly in the top choices of available rounds out there by the better known ballistics guys.

The .223 loading is? I thought the 5.56 loading with the bonded bullet was the preferred ammo for LE work?

 

IMI is the Isreali Milsurp ammo right? Generally it is labeled as "XM" in this instance "XM855". The "X" denotes it was a rejected lot. I wouldn't trust that to trouble shoot a BCM. That just me though.

 

http://www.thegunzone.com/556xm.html

IMI is not rejects. It's military ammo. It's better than US Military ammo.

 

The reason US ammo is "XM" is because it is illegal to sell US Military surplus ammo (Thanks Clinton!!!).

 

The "X" does NOT denote rejected ammo.

 

It's just made specifically for the civilian market. If it was all rejects, then Lake City would have double digit failure rate in their production. They produce way too much XM ammo for it to be rejects. Heck, Lake City doesn't even make ANY M193 anymore (Israel does), so there is no way there XM193 is exclusively rejects.

 

A small portion of the ammo was probably made from pulldown components that failed mil spec for minor reasons. But certainly not the majority of it.

 

There was a time when Lake City was producing hardly any XM855 for the civilian market back in the early 2000s and a lot of the "XM855PD" did indeed look like it had a high percentage of rejects. Those days are long past.