Proper gas port size for a 10.5" barrel?

General Firearms NFA
1

I jsut received my tax stamp for an SBR and now shopping for a 10.5" barrel. I am getting different gas port sizes and was wondering if there was a “standard” gas port size for a 10.5" barrel?

I was looking at a used Compass Lake barrel which has .069-.073 gas port size.

Spikes tactical mentioned they have gas port size between .073-.089 in size.

ADCO said they can do any size.

I am leaning towards ADCO barrels since they have a decent reputation and good bore concentricity. (Plan on using a suppressor in the future) They also come crowned and “machine gun grade”.

2

If I were to go 10.5, I’d go with LMT. They’ve got a pretty good reputation for reliability.

3

I have a BCM 11.5" and I believe that the port size is .076. IIRC Colt and LMT 14.5" are around .063. I believe that Colt Mk18’s are around .068.

I would personally go with a BCM barrel or a Noveske. I would not buy a used barrel and I would not buy a Spike’s.

4

I would like to go BCM, but they do not offer 10.5". Any particular reason why you would not go with a Spike’s Tactical barrel?

I guess I should also mention it will be going on an Ar with an Adams Arms piston kit. So if I went Noveske, it will not be their SS barrels.

5

Yes, for one my experience with Bravo Company and the fact that Spike’s still has not shown that they HP/MPI test their barrels.

I think that you would be perfectly happy with an 11.5" barrel. I am sure that you will not notice 1".

6

If it were me I would go no larger than .071" on a 11.5" barrel.

7

At the price of BCM’s 11.5" I could also pick up a Noveske 10.5" for the same price without having to get into contact with the ATF to change the dimensions of my registration. Especially, since everything took 6 months. So if I can stick with 10.5" I hope to stay at that length.

I will throw Noveske’s CMV barrel onto my potential list.

8

That’s a non issue. You can let ATF know via written notification if you make a permanent change to your SBR length or Caliber, but truely… no one gives a shit unless you’re involved in some other criminalistic endeavor.

9

Would I just write to the ATF NFA office that I am changing it? No forms to fill out?

Just be like “Hey I would like to change my SBR from 10.5 to 11.5?”. In a more formal manner of course. I assume I will received a letter confirmation or some kind of paper work to officially show the change of length?

10

This just made any discussion of gas port size completely irrelevant, the correct gas port for piston conversions is going to be different than DI.

11

According to AA, their kit will run with a barrel that is ported for DI without modification. And no modifications/changes are needed.

12

I don’t know of a piston that needs as much gas as DI, I think the piston conversions are probably all over gassed unless they have a barrel length specific gas plug or gas block that regulates the gas.

Discussing the optimal gas port size for DI cannot be carried over to a piston conversion.

13

This is straight from Adams Arms Sales and Marketing pertaining to proper gas port sizing.

14

not sure about the piston setup, but my sbr is a lmt 10.5,"DI and whatever gas port size it came with has been perfect. no reliability issues at all. not trying to go off-topic, but any particular reason for going with the piston setup? i only ask because for me, the DI setup has been nothing short of awesome. works no matter what, suppressed or unsuppressed, no matter what buffer i use, etc.

15

Right you are. Drop in piston systems are generally WAAAAY over gassed (when put on a DI gun that was designed to run as a DI gun).

Every drop in piston AR I have ever shot had to much recoil. This is why I would NEVER go with a drop in piston kit.

C4

16

Of course it will “run,” but is less than the optimal setup.

C4

17

LMT uses one of the smallest GP sizes of all the 10.5’s out there.

IMHO, there is ZERO reason to go with a piston system (especially a drop in system).

C4

18

Personal preference. But mostly because it will run cleaner.

19

Roger that, but you will STILL have to clean and lubricate it. Remember that there is NO FREE LUNCH. So there are trades being made here.

The down sides are:

  1. Weight
  2. Cost
  3. Increased recoil
  4. Less accuracy
  5. Spare parts availability
  6. Carrier tilt/wear

C4

20

An upper built around a piston can make sense on a suppressed 10.5" gun - of those the LWRCi 10.5" and LMT MRP 12.5" Piston (HK416 10") are the only ones I’d consider.

That said, my personal choice is a DI 10.5" barrel, and I’ll be running an OPS M4S or 14th can on it at all times.