Hey all.  I am having a strange problem with a 20" BCM upper and BCM bolt carrier group.  I put this rifle together last spring and have put around 700 rounds through it.  Today I go  out and after 15 shots, it begins to short stroke.  I figure it is dirty or dry (even though there are only 15 rounds since cleaning and oiling) so I pull the bolt, wipe it down and re-oil with LSA.  I have used LSA on all 5 of my AR's and it seems to be good stuff.  Anyway, still short stroking and finally no-stroking.  The damn thing is a bolt gun.





I'm not one to be discouraged so quickly so I pull the BCG from one of the others (it is the same  but M16 surplus) and the gun functions flawlessly for a mag dump.  Thinking it is the BCM BCG, I stick it in the gun I took the other one from and that one works flawlessly for a mag dump.  I put the BCM one back in the BCM 20" inch upper and the same thing, no stroke or extremely short stroking.  It so short that the shell doesn't eject but I can here a slight spring rattle in the stock so I know it has moved somewhat.





I come home, take off the flash hider, punch out the pins on the front sight/gb and pull the gas tube.  I even take the gas tube out of the front sight and it is not plugged in the least and lines up correctly.  I checked the gas hole in the barrel and it is obstruction free.  I put it back together, take it out and the same thing.....short stroke or no stroke.  For shits and giggles, I have brought a third rifle with me and swap the carriers.  Again, both rifles function flawlessly with the BCG's swapped.  I am at a total loss.


 The stock on the offensive rifle is an A2 and it has the proper buffer and spring.  I can only surmise that a perfect storm of tolerance conflicts has overcome this upper, barrel and BCG but am at a loss to explain it after so long a period of flawless operation.  





I have just left the BCM carrier in one of the rifles that liked it and swapped it's carrier into the BCM upper group.





Any insight would be appreciated........

 

This may be the problem. Different components on the high and low sides of the tolerance scale. In any case, if the parts are in spec, they should work no matter where they are on the tolerance scale.

wow!
BCM upper and BCM bolt carrier group have issues, never would have thought that.

someone will come along and tell you that you should have got a Colt

contact BCM , most members here say they have world class service

Exactly, hence my frustration and confusion as to why they will function with different parts but not any longer as a whole.





 

If you pull the charging handle and let it down slowly does it feel like it is hanging up? I have a AR that got worse as I shot more and started short stroking around the 3-400 round mark. I dinked around with it until I realized that the bolt/hammer contact area had managed to damage itself with use and was gauled. I measured the height of this hammer with the upper off and found it to be 1/8" higher than others. Tolerance stacking is to blame since I could swap things around with other guns and it works fine. I eventualy just ground the hammer until it worked well and replaced the bolt carrier that was damaged.

You know, I thought about the hammer hanging up the bolt carrier.  I haven't put a caliper on it to check the height and the bottom doesn't appear to be galled but I will definitely check tomorrow.  I have a couple spare hammers laying around and may try swapping one of those.

I have a 16" BCM SOCOM upper and BCG  that is supposed to be here tomorrow.  I'm going to do the switcharoo with these as well and see if there is any difference.



Thank you, it was something I had thought of but dismissed and will now check.

Let Bravo Co. know, they will take care of it.

Check your gas key and gas tube for an obstruction (like a dislodged primer stuck in the gas key or something like that)

Sounds like there is a serious restriction in your gas system.

It is not common, but it does happen.



Woops…
Should have read entire post first.

Good luck with a solution.

He did check his gas tube, and found no obstructions. Even so, if there is an obstruction in either the gas tube or the gas key, neither of the rifles would've functioned correctly when he swapped their BCG's.

Were you shooting under powered ammo?

+1

What kind of ammo?

In before the BCM bandwagon saying "It can't be, not BCM!".

I would suspect the ammo. I have a rifle that does the same thing, Wolf ammo it is a single shot, XM193 it runs like a top. I have come to accept that I cannot shoot Wolf out of it.

I know that's a common mantra here (the "just send it back" thing), but there's still a few things he can experiment with himself to rule out an easy-fix error.  


Were you using only 1 type of ammo throughout these experiments?  What kind?

Thanks for the replies guys.  I tried some xm193, some IMI m855, some stuff I have loaded using 25 grains of WC844 and 62 grain milspec steel core bullets, and I even tried wolf once I switched the carriers.  They all 3 functioned in the switched configuration with the different ammo's.



 I got the SOCOM 16" upper and BCG today so I am off to try it out and switch the carriers as well.  I have also switched the hammer out of the gun that isn't playing nice.  I'll report back later.  I agree with one of the above posters that I would rather fix this myself than send it in.

Thanks all.

Well......the new BCG worked fine in it and the Offensive carrier worked fine in the SOCOM upper.  I swapped them back and the gun fired 10 rounds and began short stroking again.  I've decided to just switch the carriers and be done with it.  I don't want a rifle configuration I can't trust.  I'm going to try and put 500 through the various rifles  this weekend and see how that goes.  Thanks for all of the suggestions.



Oh yeah, the BCM hat I got today was OD green , the last one was desert tan.  I'm glad they send a variety.