Colt 6940, conversion into reliable SBR w/wo suppressor

**masakari** · 12-26-14, 09:08

I have a 16" Colt 6940. It is for home defense and sometimes for patrol. I want to shorten it up, but also use it with or without a suppressor.

I have read a lot about the issues with suppressed SBRs, especially 10.5" DI guns. For this reason, I would want to send it off to ADCO, SAW, Grant (does he do that?) or another reputable shop to be cut to 11.5" and the gas port re-sized appropriately.

The can would be a SilencerCo Specwar 5.56 with Trifecta flashider. My buffer is currently an ST-T2, which I would like to keep as all my rifles use the same one. I use Rainier Arms Raptor charging handles, which unfortunately do not have the gas bleedoff channels like the Gas Buster. Sure wish they did.

Would it be plausible to think that it can be done and be reliable both with and without the suppressor? Any recommendations on gas port sizing, or should I leave that to the shop it is sent to?

Thanks for the help!

**markm** · 12-26-14, 09:30

Originally Posted by masakari

Would it be plausible to think that it can be done and be reliable both with and without the suppressor? Any recommendations on gas port sizing, or should I leave that to the shop it id's sent to?

Absolutely. Send it to Specialized Armament, not ADCO.

They do this stuff all the time.... and correctly.

**tom12.7** · 12-26-14, 10:11

Originally Posted by markm

Absolutely. Send it to Specialized Armament, not ADCO.

They do this stuff all the time.... and correctly.

+1 on this, Ken can't really be beat for this.

**SpartanDCI** · 12-26-14, 23:17

I usually suggest trading or selling your factory barrel for a Daniel Defense 11.5 inch barrel or if you can find one, a Colt 11.5 incher to go with your existing parts. You will have to make sure you have a proper headspace on your bolt/barrel, but it may be less expensive than cut and milling a new gas port. Either way make sure you have the current extractor bumper or o ring and lube it properly until it is broken in.

My old agency tested the Colt 14.5 inch carbine (727) and Colt 11.5 shorty back in the later 1980's. The 14.5 incher beat the shorty hands down for reliability, but I think it has more to do with the fact that the 14.5 incher was well broken in and the 55 grain ammo did not make a big different in timing on the 1/7 twist barrel. The 11.5 inch shorty was brand new and did not go through a single mag without a feeding issue or hammer down on empty chamber. The colt rep swapped out the 11.5 inch upper for a 14.5 upper and the gun ran fine. I bought the 11.5 inch upper from the Rep for a small sum and he suggested lubing it properly and firing some Milspec SS109 ammo, which was hard to get at the time. Jim Frigiola was an Olin rep at the time and offered my shooting buddy a small supply of Olin's heavy 5.56 ball round known as the "Penetrator". That 11.5 inch upper fired trouble free from then on mounted on my civie legal M16 carbine lower. It was extremely accurate out to 300 yards (range limit) and except for the loud blast, was easy to shoot (although it did recoil slighlty more).

I am currently building a buddy in the Navy Special Ops community a special Mk18 pistol for his VA CCW permit, and when test firing the 10.3 inch barrel I had the same sort of stoppages. A Navy armorer loaned me a Knight Triple tap comp and although the flash is great, it functioned 100% so opening the gas port or adding a bore constriction/muzzle comp does help.

ADCO can cut down your barrel and open the port as they do it all the time. They can also dimple the barrel for cooling or coolness.

Ken Elmore is a pro, but he is usually back logged and he may not be able to get to your project quickly.

Good luck either way.

**zb39** · 12-27-14, 09:31

What about buying a 6945 or 6943 upper for your colt after the paper work of course.

**ssgjason** · 12-27-14, 09:33

This is exactly what I am considering doing. I have a 6940 that I would like cut to 6945 configuration.
SAW is where I would prefer to send it.
If you go this route, update as you go.

**MistWolf** · 12-27-14, 09:58

Cutting an M4 barrel to 11.5 inches puts the muzzle right in the middle of the notch

It isn't that difficult to get a 10.5 inch upper to function reliably. Mine uses a .082 inch gas port which may be a little on the large size, but it 100% reliable with a carbine RE and an H3 buffer or rifle RE and rifle buffer. I'll be experimenting with an adjustable gas block next

**masakari** · 12-27-14, 10:32

Originally Posted by SpartanDCI

I usually suggest trading or selling your factory barrel for a Daniel Defense 11.5 inch barrel or if you can find one, a Colt 11.5 incher to go with your existing parts. You will have to make sure you have a proper headspace on your bolt/barrel, but it may be less expensive than cut and milling a new gas port. Either way make sure you have the current extractor bumper or o ring and lube it properly until it is broken in.

My old agency tested the Colt 14.5 inch carbine (727) and Colt 11.5 shorty back in the later 1980's. The 14.5 incher beat the shorty hands down for reliability, but I think it has more to do with the fact that the 14.5 incher was well broken in and the 55 grain ammo did not make a big different in timing on the 1/7 twist barrel. The 11.5 inch shorty was brand new and did not go through a single mag without a feeding issue or hammer down on empty chamber. The colt rep swapped out the 11.5 inch upper for a 14.5 upper and the gun ran fine. I bought the 11.5 inch upper from the Rep for a small sum and he suggested lubing it properly and firing some Milspec SS109 ammo, which was hard to get at the time. Jim Frigiola was an Olin rep at the time and offered my shooting buddy a small supply of Olin's heavy 5.56 ball round known as the "Penetrator". That 11.5 inch upper fired trouble free from then on mounted on my civie legal M16 carbine lower. It was extremely accurate out to 300 yards (range limit) and except for the loud blast, was easy to shoot (although it did recoil slighlty more).

I am currently building a buddy in the Navy Special Ops community a special Mk18 pistol for his VA CCW permit, and when test firing the 10.3 inch barrel I had the same sort of stoppages. A Navy armorer loaned me a Knight Triple tap comp and although the flash is great, it functioned 100% so opening the gas port or adding a bore constriction/muzzle comp does help.

ADCO can cut down your barrel and open the port as they do it all the time. They can also dimple the barrel for cooling or coolness.

Ken Elmore is a pro, but he is usually back logged and he may not be able to get to your project quickly.

Good luck either way.

Unfortunately, the 6940 series uses a unique barrel extension and nut, so simply swapping in another barrel won't work.

Originally Posted by zb39

What about buying a 6945 or 6943 upper for your colt after the paper work of course.

I would love to, but the uppers cost more than a complete 6945/3, which is outrageous. If they were sub $1000 then yeah, but at $2k not going to happen. Sucks.

Originally Posted by MistWolf

Cutting an M4 barrel to 11.5 inches puts the muzzle right in the middle of the notch

It isn't that difficult to get a 10.5 inch upper to function reliably. Mine uses a .082 inch gas port which may be a little on the large size, but it 100% reliable with a carbine RE and an H3 buffer or rifle RE and rifle buffer. I'll be experimenting with an adjustable gas block next

yeah I don't mind it being in the notch... Maybe I would have them turn the exposed barrel down, maybe I'd leave it M4 profile, maybe I'd make it slightly longer to have the whole notch available for M203 mounting. The latter is unlikely though.