Was doing some filming at the range the other day, had both the Colt LMG upper and a newly acquired Colt IAR upper out, each on on a full auto lower.

Colt LMG upper always runs 100%

Colt IAR upper will run on semi just fine, but gets hammer follow in full auto, max 2-3 rounds before a dead trigger (i.e. hammer follows).  

The IAR upper was being run on a carbine length buffer tube with I believe (will have to confirm) an H3 buffer.

Swapped the Colt IAR upper to the lower we were running the COlt LMG upper on, rifle length buffer tube and hydraultic buffer.  Exact same thing, dead trigger after max of 2-3 rounds on full auto, runs fine on semi.

Swapped the bolt carrier groups between the two uppers, tried on both lowers, still get a dead trigger after 2-3 rounds.

I know it is a timing issue of some sort, any suggestions beyond what I have already tried?   this one has me a bit stumped, and being the upper is an original IAR upper, I don't really have options for a tunable gas system, etc.


We only have the two full auto lowers to try it on, but I can try some other bolt carrier groups, and I can always switch buffer weight on the carbine lower and go to a standard rifle buffer and spring on the rifle lower.


Sven
Manticore Arms

Correct, the problem follows the upper, and is independent of bolt carrier group and lower receiver setup, and i would agree it is bolt bounce.

Upper is brand new and unfired so I don't think gas port erosion is an issue, but overbored gas port I guess could be a theoretical possiblity.



Yeah, all I had at the range was the green spring and hydraulic buffer so I do plan to give a standard rifle spring and rifle buffer a go.  I also need to actually weight the carbine buffer to make sure it is an H3 and not just a standard one, that would be embarrassing.  

I thought about doing the Vltor A5 if it comes down to it, hopefully it just takes a buffer adjustment to get it dialed in.

Thanks for the suggestions, and confirming what I am thinking the issue is, just nice to get a second opinion.  

Sven
Manticore Arms

@M4-Pilot

Great thought, it would not surprise me if that was the issue.  I will look into that.

Sven
Manticore Arms

Turns out somehow a standard carbine buffer had gotten into the lower, and a spring of unknown origin.

H3 buffer and new spring in order, hopefully that solves the issue.

Sven
Manticore Arms

Gave another go with the Colt IAR upper, this time with an H3 buffer and new recoil spring.  Ran like clockwork, no issues at all.

Sven
Manticore Arms

@KitBuilder

If I had a bone stock Colt LMG and Colt IAR in front of me, I would take the Colt LMG (closed bolt is what I am referring to).

The Colt LMG is a little heavier, but the balance is fairly "even" whereas the Colt IAR is a bit nose heavy IMHO.

The real special sauce of the Colt LMG however is the combination of the rifle length recoil system and the hydraulic buffer.  The recoil is just the smoothest, most controllable leisurely 600 rpm rate of fire.  It is the most controllable 5.56 full uato magazine fed weapon I have ever fired, and you really appreciate that it was built for very controllable and consistent full auto fire.

The Colt IARis really nothing more than an M4  carbine with a big fat hunk of finned aluminum bolted to it.  Is it more controllable than a standard M16 or M4 carbine?  Well yeah, because it has a spare pound of weight bolted out front to keep the nose down.  

Colt has not produced any IAR's, the ones that are out there are the 200 or so odd ones built as prototypes.  Mine did not come with a manual, and I am not aware of a Colt IAR manual that is out there.

I don't know if Colt Canada is producing LMG uppers, I heard they were still making them in the late 1990's, but no info that I am aware of beyond that.

Now that I have it running, I am hoping to do some videos on both of them for the Manticore youtube account, along with videos on some other firearms and tops.  The goal is to start having those up by early summer.

Sven
Manticore Arms