need help, the pros and cons of having the gasport at 12 or 14" on AR15

[Michaël T]

i own a AR15 with the gasport at 14" and 28" barrel length, the accuracy for a 5 shot group is 1.55" at 200yards with 107 smk jumping 0.020" going 2850fps, 29.0gn varget lapua brass and cci450
question is would i be better off with the gasport at 12" and the barrel length at 26"
i have tried seating depth in and out of lands( 0.010" in to 0.025" out) from 2700fps to 2850fps

thanks to all and have a great shooting in 2013

 

[jonbearman]

I think you would be best served with an adjustable gas block to soften the recoil.

 

[mugadutz]

so what is your problem i have 3 ARs if i could get those results i would be happier than a witch in a broom factory
just my 2 cents

 

[Cort]

Contact Robert Whitley (RCW3 here on the forum). I believe he has forgotten more about making an AR shoot than most of us will ever learn.

My opinion is that the length of the barrel and the position of the gas block is not as critical as the quality of the barrel and the care taken in installing the parts in the upper. I don't think you will gain a thing by shortening the barrel or changing the position of the gas block.

Cort

 

[Michaël T]

Jon, why would you think that i have a recoil problem.
gun weights 14#, all load development was done with frt, rear bag and sled,scope measured over oehler 35p at 200 yards.

 

[placek59]

I believe he meant the recoil of the gas system - it may be "over gassed" putting excess pressure on the bolt and carrier (all the moving parts) before the bullet leaves the barrel. I don't think he meant that the gun kicks too hard. At least that is what I have been told as the main reason for an adjustable gas block. It "smooths" out the cycling, even though it may cycle just fine, it might be more "violent" than it needs to be...? Clear as mud? Maybe someone can explain it better...

oh, and by the way, what caliber? just curious.

 

[jonbearman]

What he said,that is what I meant as I have 5 ar's and have some time under my belt,just explained it wrong.

 

[Michaël T]

it's a 6mm fat rat with a 28" satern barrel with the gasport at 14"heavywall upper, sidecharging handle

 

[mac86951]

I can't vouch for it personally, however, I do believe that accuracy and velocity tests were done with an AR where the
gas-tube/front block was removed. Velocity didn't change from a fully assembled AR, and accuracy didn't suffer either.

That said, I do know of some ARs in odd calibers such as an AR-10 in 7mm SAUM. Moving the gas-port length was required
to lessen the recoil, not to the shooter as much as reduce how the bolt is being driven. The same effect can be duplicated
with an adjustable gas-port.

Why do I bring up recoil? Well, again, theoretically if you are not near complete on burning powder, a shorter gas-tube might
be extracting the case too soon. This might affect accuracy by introducing some velocity variation, which could be checked by a chrono, or even by trying a different powder.

In practice, I've yet to see an AR that drives the bolt too hard with gas pressure only show an accuracy problem first. With poor accuracy, I'd try different bullets/powders/velocites (see Ladder Testing) and perhaps your ammo is not in a "node" for your rifle? A chrono can verify if this is the case as well as ladder testing. Another possibility is that your recoil has loosened the barrel nut? Enough that it might be touching one side of the gas tube? I've seen a loose barrel nut be a problem before.
This of course assumes that you've already checked the sights to ensure they are tight, and you've done the shooting off of bench and bags, or sled.

-Mac

 

[Scott Harris]

... Why do I bring up recoil? Well, again, theoretically if you are not near complete on burning powder, a shorter gas-tube might
be extracting the case too soon. This might affect accuracy by introducing some velocity variation, which could be checked by a chrono, or even by trying a different powder.
...
-Mac

Both gas port location and gas port size matter. For best accuracy, you want to delay movement of the BCG for as long as possible....ideally until the bullet has left the barrel. For this reason, moving the gas port from 14" to 12" would be going in the wrong direction. Given you are using a wild cat, it's anybody's guess as to the correct port size and location for optimum function/accuracy. Robert Whitley would be the first person I'd contact.