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

[grey2112]

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?

[320pf]

grey2112/JohnCrighton,

I put the gas port in the pistol position in all of my 300-221 fireball rifles. If you are going to run a suppressor you can probably get your rifle to run with a 0.078" gas port. With an unsuppressed rifle you will probably need to open up the gas port to about 0.083".

The best port diameter depends on the burn rate of the powder that you use. I shoot my 300-221Fireballs with both sub-sonic and super sonic loads and have a fixed gas block. super sonic load is 19gr W296 pushing a Speer 125g TNT at 2250 fps. For my subsonic loads I usually shoot 165 to 180 gr bullets infront of 8-9 gr of AA#9. I am considering trying to develop a subsonic load using W296 because it is easier to get where I live.

320pf

[grey2112]

Thanks, 320pf - so, if I wanted to do both (shoot with and without the suppressor) on the 16" barrel, with an adjustable gas block, should I go with a .083" port diameter? This way I could use the adjustable block to turn it down for the subsonic shots and open it up full for the supersonics?

But regardless, I should stick to pistol length for the 16" barrel, correct? Even though I'll be using an adjustable block.

[320pf]

Thanks, 320pf - so, if I wanted to do both (shoot with and without the suppressor) on the 16" barrel, with an adjustable gas block, should I go with a .083" port diameter? This way I could use the adjustable block to turn it down for the subsonic shots and open it up full for the supersonics?

But regardless, I should stick to pistol length for the 16" barrel, correct? Even though I'll be using an adjustable block.

You can probably get the rifle to run without an adjustable gas block. That is what I would work for first. Adjustable gas blocks are a pain so if you can get you rifle to run without one you are better off.

320pf

[LouBoyd]

You can probably get the rifle to run without an adjustable gas block. That is what I would work for first. Adjustable gas blocks are a pain so if you can get you rifle to run without one you are better off.

320pf


My choice would be to tune the first rifle for a specifc subsonic load for a bullet of choice (240 SMK or similar). I just don't see the point in a supersonic 300 Whisper. The same action and barrel can handle a larger cartridge like the 7.62x39 or 30AR (6.8 SPC necked up to 308). At least you can get some range before it goes transonic.

Yes, put the gas port on the subsonic at pistol length unless you want the hassle of making your own gas tube. I'd tune the short barrel by slowly increasing the port in the barrel using a standard gas block and gas tube. It's easy to make the barrel hole larger in small steps with a numbered drill set. I prefer to limit subsonics to 1000 fps or even 950 fps.

More greatly increases bullet noise and upsets stability even though it's technically subsonic.

[Fudmottin]

My barrel maker was kind enough to test fire my barrel so that it would cycle with the gas port in the pistol position and an adjustable gas block that has three positions: High, Low, Closed.

I haven't busted the cherry on it yet. Been too busy. But when I do, the first person to get feedback will be my barrel maker. I will probably also post here.

BTW, my barrel is threaded in the 5/8-24 standard for 30 caliber barrels. That makes getting a commercial can much simpler.

[steezo757]

I'm also trying to decide the proper gas port size. I have a Noveske 10.5" barrel with the gas port in the pistol position. And have not been able to work up a load that's accurate and cycles, so I had a local gunsmith in VA (when I lived there) open up the gas port to 0.107". I've read recommendations that anywhere from 0.085" to 0.125" will cycle with subsonic rounds.

First off, are there gas tubes with a I.D. of 0.125"? I thought the normal gas tube I.D. is around 0.110". I'm thinking about having a 3 postion gas block made by tp555. From what I understand I can select two gas port sizes with the 3rd position being off. I want to run only subsonic. Does anyone have any recommendation for the two gas port sizes? Again, I only want to run subsonic.

This AR platform is becoming a little fustrating. I'm also working on a .300/.221 bolt gun and am considering just focusing on that and deal with the AR later.

*Sorry for the hijack, but I didn't want to start another GAS PORT thread...

[rufracer]

I have a m1s 16" upper with a jp adj gas block on a cavalry arms lower, which has a rifle length buffer tube, and it works well with all supersonic loads ranging from 145gr w/18grh110 to 200gr w/17gr 1680, and subsonic loads 200gr w/12gr 2015. and 10gr 1680. The jp gas block works fine, if you get one, put a little teflon tape on the adjustment screw to keep it from backing out.

[grey2112]

Does anyone have the measured length of a pistol length gas tube for an AR? I need to cut down a mid-length to 7/8" less than a pistol length but need to know the length of the pistol length first before knocking another 7/8" off it.

[tp555]

Length is 6.7".You can cut down 3/4" and install to see how much slack you have.BC should not touch the tube and you can have 3/16" extra space.I once made barrels for a guy that was 3/16" farther away from the stock location.

[grey2112]

Thanks, TP - I had to drill out the hole in the gas block on both barrels to get the tubes to fit - VERY tight fit and hopefully I got them deep enough - will know soon when I test them out.

[tp555]

Shouldn't have to drill the blocks.They have a good no slop fit .I test with 2 gas tubes.Your gas tube would be the problem and could be too big.Gas tube is .180" dia.If you drilled it you will have gas leaks.A # 15 reamer is what I use for that hole.

[grey2112]

Shouldn't have to drill the blocks.They have a good no slop fit .I test with 2 gas tubes.Your gas tube would be the problem and could be too big.Gas tube is .180" dia.If you drilled it you will have gas leaks.A # 15 reamer is what I use for that hole.

I used three different gas blocks from 2 different manufacturers and had this problem - literally had to bang them into place with a hammer.