16" Colt 6940 upper to 11.5"

AR/M4 AR Technical Discussion
1

I have a Colt 6940 upper with the 16" barrel. I am thinking of having it cut to 11.5" for a pistol build maybe to become a SBR someday.
I don’t know the gas port size of the 16" Colt but my question is, will it work as an 11.5" barrel?

Thanks

2

I think the carbine has a .059" port. A good 11.5 would be .069 to .071 MAX!

Now if you ran a suppressor on it, it might work ok.

3

I think the 16” has the same port as the 14.5”, .062”-.063”, but I’m not certain. It’s unlikely that port size will run a 11.5”, but it works great on a 12.5”. My current main beater at home is a 12.5” Hodge with .0625” gas port, and it runs any brass ammo unsuppressed and any ammo including Tula with a can.

4

So it looks like I’ll have to have the port size opened up. I don’t have a can but I won’t rule it out in the future.
Thanks all !

5

Minor port reaming is a simple task if you have a drill press. Reamers aren’t very expensive.

6

I know I started this thread about two years ago but I thought I would follow up. So I went ahead and did it, I had the 16" barrel trimmed to 11.5”. It runs just fine, very soft but, so far, 100% reliable with all 5.56 ammo I’ve put through it. I’m using an H buffer. At first it sat on my pistol lower, now on my SBR’ed lower. Aimpoint Comp M4 with folding rear iron to go with the Colt folding front gas block.

Just awesome !

4 Likes
7

Sounds good. If SBRs ever come off the NFA my 16” guns are going to 12.5”.