Educate me on gas tube length, gas block holes, etc.

[Greywolf]

OK, have a technical question. I am getting two barrels made for my AR-15 in .300 - each will have its own upper, handguards, etc. - basically two complete uppers.

#1 is a 8" barrel, 1/8 twist, with DD Lite handguards on an ASA side-charging upper. It will be almost exclusively used for subsonic use with a suppressor.

#2 is a 16" barrel, 1/8 twist, probably with DD Lite or Omega handguards on a regular M4 flat-top upper. It will be mostly for supersonic use (2400 fps 125 gr bullets mostly) though I'd like to be able to use it for subonics as well occasionally with my suppressor.


The question is this - what length gas tube should #1 and #2 get, what diameter gas port hole do I need, and will I gain benefit from having one or the other or both have an adjustable gas block?

I've already paid a member here to make the barrels and also paid for the adjustable gas blocks.

But I want to make sure that the gas hole is sized correctly for my needs for both rifles, that the position is the proper one, and that I really do need either or both adjustable gas blocks.

What say you all?

 

[RyanM]

They'll both need pistol gas tubes to work with subsonic ammo (you don't have a choice on an 8" barrel anyway). And an adjustable gas block is a necessity for running both subsonic and full power supersonic ammo.

You'll most likely also need 2 different subsonic loads, as one that's 1050 fps from an 8" barrel will be supersonic from a 16", and one that's 1050 fps from a 16" will probably only do 800-ish from an 8".

Gas ports, I have no idea.

Let us know how well done the barrels are. I'm planning on having a 6.5" to 8" AR in .300 whisper made at some point in the distant future, and am still shopping around.

 

[Greywolf]

Thanks, RyanM - so both in the pistol position then, and two different loads - not a problem. I assume that when using subsonics I'll have the gas block opened up full to make the use of the most gas, and when using supersonic I'll have it half-closed, right?

Luckily with the side-charger I can even use ammo that might not cycle the bolt and I'll just cycle manually.

 

[ants]

Have you tried the AR sites, Wolf? That's where I go for specifications on 30 cal builds. I've never done subsonic so I can't help you there, but the AR15.com crowd will probably have your answers. Wade through the thickheads and you find very nice gentlemen who graciously help a humble little guy like me.

Just my guess for gas port diameter: Match the hole in the gas block. That's what I did.

 

[RyanM]

Just my guess for gas port diameter: Match the hole in the gas block. That's what I did.

Yikes! I'm almost positive that's too big, except for an adjustable gas block.

FWIW, Colt's gas port sizes for 5.56s with CAR length tube are:

10.5" barrel - 0.093" (0.093" is also the size for a 20" with rifle tube)
11.5" barrel - 0.081"
14.5" barrel - 0.063"
16.0" barrel - 0.063"

A lot of manufacturers open their 14.5" and 16" ports up to 0.07", or even close to 0.08", for better reliability with light bullet .223 ammo (probably also why Colt's 16" has the same gas port size as their 14.5", the military doesn't buy 16" ones).

I assume that when using subsonics I'll have the gas block opened up full to make the use of the most gas, and when using supersonic I'll have it half-closed, right?

Something like that. The easiest way for the barrelsmith to do it would be to just make the port intentionally oversize (even the size of the hole in the gas block), and make you do trial and error test firing to see how much the gas block needs to be opened up for reliable function with each type of ammo.

You may also need to open the gas block a little if you take the suppressor off, as that will reduce gas pressure.

What can are you planning on using? I'm still shopping around on those, too, but I'm thinking of trying to find a very compact .308 one, so it'd have no problems with full power 125 gr loads. The sonic crack is no louder than a .22, so it'd still be a pretty quiet rifle. It'd be nice if the barrel length with can is still under 16", so that I don't "waste" an SBR tax.