GAS SYSTEM length on an ar15

[mike in co]

ok lets get off the other thread and disscuss gas systems on an ar15 platfrom.
sub guns(under 16)
16...m4 etc
mid longer than 16/shoter than 20
20's
longer than 20/out to 24
and then longer than 24.
i just started on my first 2 16's( a 5.56 and a 7.62x39)
i have several std 20's
and one 26 with the gas block moved to plus 2"
it is a 1/14 with just 52/53's at 3500/3600fps.....works well.

your turn...this is a new area for me....

[klcarroll]

Good! …..This is a subject I want to learn more about!!

I currently have two .223 and one 5.56 caliber weapons (two Rugers and one “Shorty AR”): ……And all three work just fine with factory duplication loads; …..But I would like to depart from that a bit.

Since “Social Shooting” is (happily) not currently one of my big concerns, I would very much like to build a weapon that has the ability to function well over a broader range of reduced loads than any of my current weapons will accept.

My reasons for looking at loads in this category are mainly economic: I want burn less powder, make my barrels last longer, slam the action around less harshly, and contribute less to my Tinnitus with each shot. (….And I don’t think the woodchucks will say; “Thank God that wasn’t a Full Power Factory Load!!)

A good example of such a load is 14 grains of 2400 under a 55 grain jacketed projectile: …Accuracy-wise, it shoots well out of all three of my weapons, and makes considerably less fuss and bother at the muzzle; …..But in spite of the fact that it produces 2550 fps out of an 11” tube, it WILL NOT function the action of any of my toys! (Irritating!)

My goal here is to configure my next AR build so that it will tolerate (or be adjustable down to) this load level: ……And I figure that an intelligently laid out gas system will be a big part of that solution!

I will be following this thread with interest!!

Kent

[Storydude]

Lighter loads need a lighter buffer, if gas length is unchanged.

Pistol powders don't like the increased volume of the gas system, in the AR platform. They run out of oompfh before they can operate the action.


Those are a good choice for a piston system, with an adjustable gas block. Tuning an AR is kind of difficult because a very light load that operates the action, without an adjustable gas block, will batter the action with standard loads. You can change buffers to help, but not eliminate the differences.

[klcarroll]

.....Those are a good choice for a piston system, with an adjustable gas block.

That's an interesting suggestion! ......I hadn't considered a piston setup.

Kent

[Storydude]

Hey, I have something that was once yours

[Moonie]

For lighter loads I would suggest a few options. If building new use the carbine length gas system, drill the gas port larger and use an adjustable gas block. Using the short length gas system will give you higher pressure and longer dwell time but will also give you more recoil than a mid or longer length gas system. The adjustable gas block will still give you the ability to shoot full power loads with a simple adjustment.

I currently have a mid-length AR 15 in 5.56 and look forward to playing with the Bator mold if it ever shows up but I will be tailoring the load to what will cycle the action without the above modifications.

I am also building an upper in 6.8 SPC and will be casting for it, seems that is uncharted territory but I have found a 110gr .277 mold at LBT that should be perfect. Will try paperpatching with that one.

Thinking of PP'ing the .224 boolits for the AR but I believe they might be a tad small for my middle aged fingers/eyes.

[Lloyd Smale]

Im no expert on ars thats for sure but ive got 7 of them in 556, 762x39, and 50beo and an ar10 in 308 and all are 16 inch guns. All run very reliably.

[buckweet]

i feel that mid-length gas is best in a 16'' barrel.
but im also running a tubb's spring and H2 buffer, super soft shooting.

having said that' just built a super dooper lightweight rig with a pencil barrel , 20'' incher with rifle gas n' M4 stock' also with tubbs spring n' H2 buffer. in a word = AWSOME.
using the plum crazy lower. and the aluminum free floating handguard, she's one soft super accurate shooting rifle.
its so light you'll hit yourself in the mouth picking it up !

BUCKWEET

[Moonie]

Yes but with the tubbs spring (stronger) and H2 (heavier) buffer in rifle length gas system it isn't going to like light loads at all...

[madsenshooter]

Timely, I'm about to do some experimenting in this area. I have a 6x45 upper coming with a rifle length gas tube, adjustable gas block, 24" barrel. I opted to have the gas port drilled the same size as the .223, reasoning that my j-word loads will be working at the same pressure as, though the gas volume would be slightly increased, over, .223 loads. And, if I need to open it up to use the Eagan MX2-243 sometime later, I could. I probably wouldn't be using any powder faster than 4198 with the Eagan, though Blue Dot is an option too. Barrel is my $10 Obermeyer, looks to be a very fast twist. I plan on starting out with the Eagan around 2000fps with 4198 and working up. Probably around 16 gr. If someone with quickload knew the distance to the gasport, pistol, carbine, rifle, it could easily calculate port pressure with any given load. The max pressure the system can handle we know, question is, what's the minimum that will operate the action.

[Moonie]

I successfully used 18gr H4895 in my mid-length 16" AR with a light action spring using the Bator boolit. Function is flawless, brass falls at 5' at about 2:30 to 3:00.

Don't have a chronograph so I can't post velocity however.

[madsenshooter]

Do you have a standard size gas port? The fellow that was building mine wanted to put a .125 gas port in it, an idea I nixxed, seemed entirely too large for a .243 bore. I guess I'll find the proper size through experimentation. I'll probably establish the load first then work up a few thousandths at a time until I get full function with the reduced load. After that I can throttle back the gas block for full power loads.

Or, there's another way for me to approach this. I'm only going to be dealing with one cast boolit, the Eagan, and I have a quantity of ball powder that would normally be considered too slow for the 6x45. I'd certainly have enough gas volume, even if the chamber pressure was low, Lee's slow burning powder for cast theory. The powder I have is DP85, about the same as Ramshot's hunter, and similar in burn rate to RL19 or H4350. I've got CCI #41s to fire it with. I know the alloy is good for 35,000psi. I think this might work better than reduced loads of 4198.

[Moonie]

I do have a standard size gas port, everything is bone stock with the exception of the action spring.

[Artful]

thought this might be of interest in this thread...

I have AR's with carbine, middy and rifle gas tube's
I have not tried a pistol position but I saw this mod for a 300 whisper
chambered gun and am thinking 'bout mod'ing my 300 whisper.

[mike in co]

i don't think that is a good chart...
he is at max pressure
he is using a heavier bullet than normal(55 or62)
the powder seems to peak quicker.....

my 2230 accurate powder says 57kpsi for a 55 at 3100 from a 20 in bbl

which is 16 kpsi at 12 in (rifle).........not the 20k psi he shows

mike in co

[82nd airborne]

Didnt somebody bust somebody else out for putting info in the .458 socom in the "Military Rifles" forum, saying that the .458 isnt a military weapon? I never used one in the service, or an ar. But, to stay on topic, I think gas tube length depends so much on the intended use of the rifle and caliber, that it is near impossible to say what the best length is without specifics.
P.S. Mike, I'm just giving you a hard time.

[mike in co]

Didnt somebody bust somebody else out for putting info in the .458 socom in the "Military Rifles" forum, saying that the .458 isnt a military weapon? I never used one in the service, or an ar. But, to stay on topic, I think gas tube length depends so much on the intended use of the rifle and caliber, that it is near impossible to say what the best length is without specifics.
P.S. Mike, I'm just giving you a hard time.

yes but these are mil arms......223/5.56 and 308/7.62x51.....
the mil has them in all varirations of length........
our mil loaded 7.62x39 as in lake city...

so i thinking much closer than a 458 socom(find me some 458 lake city brass...)

i have had no issues yet with my 16" guns....and i know there are lots of people with lots a variarions in just these 2 calibers....was just looking for input from users...

[BD]

I do not believe that the chart is valid as it's got all of the port locations pasted onto the same rifle length pressure curve when in the real world the pressure curve is different for each barrel length.

I believe that the ideal port location is dependent on the specific load being used. When the bullet heel passes the port there needs to be enough gas volume at enough pressure to operate the action before the bullet exits, dropping the pressure to zero. At the same time the pressure needs to have dropped enough that the case lets go of the chamber walls or the case head gets ripped off.

This is a balancing act that is always effected by the bullet weight and powder speed. You can play a bit with buffer weight and carrier weights to delay the bolt unlocking a bit, and you can lighten up the buffer spring to open the bolt a little quicker, but this can only get you so far. You can limit the flow of gas through the port with a valve, but again this will only buy you a very small amount of leeway. In my experience a combination of all of these factors will allow quite a bit of variety in .223 loads. Enough to do what I want. No so with the bigger bores.

I think the only way I'm going to be able to shoot both 200 grainers at 2,500 fps, and 400 grainers at 1,000 out of the same .450 Bushmaster will be to have two ports, and some mechanism to switch between them.

BD

[82nd airborne]

Im just giving you a hard time Mike, no insult intended. I find carbine length works for alot of different variations as long as you use an adjustable block or tube, or get the right port size.
What caliber do you have in mind? Or is it just an "in general" type question?

[82nd airborne]

I think the only way I'm going to be able to shoot both 200 grainers at 2,500 fps, and 400 grainers at 1,000 out of the same .450 Bushmaster will be to have two ports, and some mechanism to switch between them.

BD

That is a very very interesting idea to me, Im going to be thinking pretty hard about that.