AR Gas system

[Gappmast]

I'm building a couple of AR15's looking for accuracy, they will have rifle length gas systems. What is the minimum distance you can have between the gas port and muzzle and still get reliable cycling?

 

[HPMike800]

I know you can run a rifle length system on a 16" carbine barrel and they work just fine. I do not know if the configuration affects gas port size.

 

[mac86951]

Curious as to what your goal is. Yes I've seen rifle length on a 16" barrel, but not sure that would be wise for 1000y. Gas system will function, but the short barrel is the trouble.

IME, accuracy from an AR is all about the barrel. Length is simply to achieve the velocity for the distance without overpressure. A longer gas system allows easier set up of gas system to not beat up brass. All else being equal, a match 16" barrel with carbine gas system can be very accurate, but the rifle length will keep my brass in better condition.

Between the two, I'd prefer the rifle length gas system.

I guess to summarize, yes the longer gas system helps in accuracy, but to the extent of not ejecting the brass early. Signs of poor timing go from case head swipe to extractor marks / broken extractor.

Handloads are harder to tune with a narrow window of operable pressure presented from a port such as a carbine length on 18", or a rifle length on a 30".


-Mac

 

[Shynloco]

Gappmast,
Slightly off topic, but if you haven't already done so, take a look at JP Precision Gas Systems. They are adjustable and there are six (I think) variations.

Alex

 

[Gappmast]

I intend to use at least a 20" barrel most likely it will be 22". I was curious as to how much dwell time is needed after the bullet passes the port and before it exits the barrel to charge the gas system. I also intend to use an adjustable gas block.

Any suggestions on port diameter?

 

[Gappmast]

Gappmast,
Slightly off topic, but if you haven't already done so, take a look at JP Precision Gas Systems. They are adjustable and there are six (I think) variations.

Alex

I'm going to use the JP adjustable block

 

[R.Morehouse]

Gappmast,
Slightly off topic, but if you haven't already done so, take a look at JP Precision Gas Systems. They are adjustable and there are six (I think) variations.

Alex

Yeah that's the way to go for sure. Real quality product....

 

[mac86951]

A 20 or 22 will be fine with rifle length. If you are going heavy barrel,. .936 barrel diameter under the block is recommended, if you are looking to carry at all, .750 will do. Plan for hand guard diameter too, I'd recommend covering the block with handguard if you're using a scope to fight mirage.

-Mac

 

[Gappmast]

This will be a target rifle shot off a bag. Im using a Lilja barrel .936 from the block forward. Along with a Seeken upper with a 2 inch wide flat bottom

 

[mac86951]

This will be a target rifle shot off a bag. Im using a Lilja barrel .936 from the block forward. Along with a Seeken upper with a 2 inch wide flat bottom

Excellent choice. That is a recipe for an accurate upper. You've already mentioned adjustable gas block, that will allow you to keep your brass in good shape too.

-Mac

 

[akajun]

try a .087 gas hole, then .089, no bigger than .093.

I'm guessing here as my 24" space guns with a rifle length gas system use a .089 gas hole.

 

[mac86951]

try a .087 gas hole, then .089, no bigger than .093.

I'm guessing here as my 24" space guns with a rifle length gas system use a .089 gas hole.

My 20" .936 barrel from white oak was overgassed at .093. I'm using 77s with N140 so powder burn rate has a lot to do with it, but if you're planning a 1:8 or 1:7, for heavy bullets, I'd recommend a 0.089.

-Mac

 

[Gappmast]

I'm going to use an adjustable gas block so I can control a over gas situation

 

[akajun]

My 20" .936 barrel from white oak was overgassed at .093. I'm using 77s with N140 so powder burn rate has a lot to do with it, but if you're planning a 1:8 or 1:7, for heavy bullets, I'd recommend a 0.089.

-Mac

.093 is pretty much standard for service rifle barrels using 77grs with varget, r15, 8208, etc. I have never found mine to be overgassed even with 24.3 hrs of r15 and a Berger 82. Must be that n140