Gas port diameter vs. erosion when cutting down a barrel

Slim · May 16, 2020

So, I have two 16” Colt barrels that don’t fit my needs. 6920 and 6721. They’re used enough that they won’t fetch collector prices, but have plenty of life left in them. With the increase in Colt prices, General lack of quality parts right now, but availability of AR machinist, I’m looking to cut them down to 10.3”.

Both barrels date from ~2009. The 6721 was a LE trade-in and had a fair (but not alarming) amount of port erosion as identified during a SOTAR armorer class last spring. The 6920 has been shot much less; zeroes and qualification mostly.

I’ve researched the “Crane Spec” for the OG CQBR/Mk18 and the rationale for the 0.070” gas port.

My question is, will a 16” barrel, cut to 10.3”, likely function without gas port mods? Both will wear a suppressor (M4SD, 30SD, SOCOM556) most of the time. Ammo will generally M193/M855, FBIT3M, or Mk262. Any unsuppressed fire will only be with duty-grade ammo.

Some obvious “it depends” unknowns are the actual extent of erosion, which I can’t really tell without (destructively) sectioning the barrel.

clandestine · May 16, 2020

It will run with a can, but without a can it may not run at all. It might run on NATO pressure ammo, but not steel cased or PMC.

Best bet it to have it cut and leave the port alone and see what it does.

Slim · May 16, 2020

clandestine said:
It will run with a can, but without a can it may not run at all. It might run on NATO pressure ammo, but not steel cased or PMC.

Best bet it to have it cut and leave the port alone and see what it does.

The man himself! :lol:

I didn’t bother you because I thought you’d be in a class. Thanks for weighing in, Chad.

Yeah, that’s what I was thinking, normal way to be sure until I get it cut. Only risk is time, and more shipping, but still worth it to experiment.

Seems like so many factory 10.3/10.5” barrels are overgassed from the beginning, and will obviously only get worse with time. I understand why commercial manufacturers do it, so they’ll run with underpowered cheap ammo, but I’m trying to avoid that.

clandestine · May 16, 2020

Slim said:
The man himself! I didn’t bother you because I thought you’d be in a class. Thanks for weighing in, Chad.

Yeah, that’s what I was thinking, normal way to be sure until I get it cut. Only risk is time, and more shipping, but still worth it to experiment.

Seems like so many factory 10.3/10.5” barrels are overgassed from the beginning, and will obviously only get worse with time. I understand why commercial manufacturers do it, so they’ll run with underpowered cheap ammo, but I’m trying to avoid that.

If it won't run and you don't have the equipment to open the port, you can always bring it to me. Next month I'll have a test trap in the shop too.

clandestine · May 16, 2020

No class this weekend. Just making gauges.

Slim · May 16, 2020

clandestine said:
If it won't run and you don't have the equipment to open the port, you can always bring it to me. Next month I'll have a test trap in the shop too.

Thanks, Chad, I appreciate the offer!

Slim · May 16, 2020

Got the barrels pulled from the uppers. That 6721 barrel was in there pretty good, but a 12” pipe on a DPMS barrel wrench got it off. Yay leverage!

Magnumite · May 18, 2020

I had a Classic DPMS 308 upper I put a free float handguard on.
I double, triple and quadruple checked to make sure I was doing things correctly.
Felt more like axle nut torque than that of a rifle barrel nut.

Mini14tac · May 18, 2020

For what it worth I had John at Duffys cut down a 16 inch CHF barrel then he threaded, crowned, pinned and welded a CAR-15 type moderator to keep it legal and it runs just fine.

Slim · May 18, 2020

Magnumite said:
Felt more like axle nut torque than that of a rifle barrel nut.

That was for putting the new one on, or getting the old one off?

Slim · May 18, 2020

Mini14tac said:
For what it worth I had John at Duffys cut down a 16 inch CHF barrel then he threaded pinned and weld a CAR-15 type moderator to keep it legal and it runs just fine.

Good to know, thanks!

I’ve got mine packed up to send to Class 3 Machining this week.

Slim · May 18, 2020

Mini14tac said:
cut down a 16 inch CHF barrel then he threaded, crowned, pinned and welded a CAR-15 type moderator to keep it legal and it runs just fine.

Now you’ve got me thinking maybe I should cut the 6721 down to 11.5”. Like the old Shorty HBAR that Bushmaster used to sell, or the FBI HRT clones...

Magnumite · May 18, 2020

Slim said:
That was for putting the new one on, or getting the old one off?

Getting old one off. I’d put one on THAT tight. Of course, maximum tongue and some thread set can cause high breakaway torque.

Slim · May 18, 2020

Magnumite said:
Getting old one off. I’d put one on THAT tight. Of course, maximum tongue and some thread set can cause high breakaway torque.

True. I’ve got a torque wrench for re-installation, but sometimes the barrel but teeth don’t line up for the gas tube clearance until you’re in the upper end of the torque spec.

ted76 · May 19, 2020

Slim said:
True. I’ve got a torque wrench for re-installation, but sometimes the barrel but teeth don’t line up for the gas tube clearance until you’re in the upper end of the torque spec.

You can buy shim washers to fix that.

Slim · May 19, 2020

ted76 said:
You can buy shim washers to fix that.

I’ve seen those used for free-float tubes, but never a standard GI Barrel nut. I guess if would work for that too, though I’d probably only try if the torque was outside the spec.

Have you used them for a standard barrel nut?

Slim · May 20, 2020

Shipped my barrels off to Class 3 Machining to cut to 10.3”. I’ll report back when I can get them to the range!

Slim · Jun 26, 2020

Barrels are back from gunsmith, cut to 10.3. Noticed that the 6920 barrel had a bigger gas port that the 6721. Both produced in roughly the same time period - 6920 has an 07/09 stamp, 6721 is 11/08.

Gas port diameter vs. erosion when cutting down a barrel

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