12.5" barrel on a mid-length gas system???

AR/M4 AR Technical Discussion
1

So basically I’ve got a BCM 16" mid-length upper and CMR 12" rail that’s not being used.

Is there any reason why it would not function if I chopped the barrel to 12.5?

2

Only one reason.

3

Thanks, very helpful.

4

I am glad I could help.

5

Dwell time. After your projectile passes by the gas port, until it leaves the barrel it is pushing gasses into the gas tube and BCG/piston. Really basically, if there isn’t enough dwell time, then it doesn’t push hard enough.

Best gas system for a 11.5 or 12.5 is carbine.

6

So a midlength 13.7" barrel has enough dwell time? 1.2" is the difference between working and not working?

Im just curious.

7

You may want to look up “dwell time”. A midlength gas system on a 12.5 does not allow enough gas to get back to cycle the weapon reliably before the pressure drops as the bullet exits the barrel. Even some 14.5 middys can be finicky with lower powered ammo.

ETA I was beaten to the explanation.

8

If you are going to get your barrel chopped, why don’t you just have adco do it and they can relocate the gas port too.

9

Is this going to be for a dedicated suppressor build? Just wondering. That changes things a bit.

Sent from my HTC One using Tapatalk

10

Sort of. You can always open up the gas port until it works. People run 16" rifle-lengths, but I would advise against it. There is little forgiveness from a reliability standpoint.

11

Robb Jensen is the man to ask. We were commenting on this the other day and apparently has has a project he has been working on with a 12.5" middy and it’s working. I’m a little intrigued to say the least.

12

IINM, Constructor has had some luck doing 12.5 midlengths as well.

13

MicroMoa, who makes the Govnah gas blocks, also makes 12.5" mid lengths and says they run great with very little recoil. Maybe he will chime in.

14

minimum I would do for a middy gas system is 14.5.

Although this is interesting.

Robb Jensen is the man to ask. We were commenting on this the other day and apparently has has a project he has been working on with a 12.5" middy and it’s working. I’m a little intrigued to say the least.

15

Salient’s 14.5" uses a rifle length gas system. If that works (and it apparently does, and very well), I don’t see why a 12.5" mid-length wouldn’t, assuming it’s put together properly.

16

They use a special gas block that causes a venturi effect.

17

Isn’t the LVOA SBR a 12.5" middy?

Sent from my HTC One using Tapatalk

18

Here’s a bit of an older thread (2011), that has responses from Robb and Todd.k

https://www.m4carbine.net/showthread.php?76076-12-5in-Midlength

19

Did it a while back (few years actually). Its possible, but you may have to open up the gas port.

IMO not worth the trouble i went thru.

20

Here’s mine. 16" BCM middy cut to exactly 13" from end of the bbl extension to the muzzle for a 12.5" bbl. I opened up the gas port .007" bigger and it runs 100% even with PMC Bronze .223 using a M16 carrier and a H buffer. Dwell time schmell time. This bbl I used to use on a dedicated suppressed upper with its original size gas port and a LMT enhanced carrier. I had it laying around so I said what the hell give it a try. I had my doubts too. Next I want to try an H2 buffer and also Wolf, Rem UMC and or Radway Green ammo to just see how what it’ll do.