AR-15 Adjustable gas block / bullet weights

[Doc7]

Hey all,

I built my first AR a few months ago and so far it has run great although that's only a couple hundred rounds of the Federal lake city XM193 ammo. I have a JP Enterprise A2 front sight with integrated adjustable gas block. I single loaded rounds while opening the gas block slightly until I achieved BCG lock on an empty magazine and then a little beyond, per instructions. I haven't had any malfunctions since the intentional ones with the port undersized.

I am more curious than anything else; when I get a chance to shoot some 69 gr SMKs (Black Hills) and 77 gr SMKs (Federal Gold Medal Match) is the adjustable gas port likely to cycle the action or not? I don't understand an AR enough to know if the increased bullet weight will make a predictable difference or not. I understand powder charges are different so it's not like we can say definitively one way or another without testing, but if you had to guess, what would you predict?

I do understand that the final upshot of any adjustable gas block system is that it has to be tuned to a specific ammo eventually (or left wide open, or at least open enough to cycle all ammo tested), but this is more for curiosity for an upcoming one-time informal group size match with a buddy of mine. He will be using a bipod and 4-16x scope whereas I may only use a sling and a 3-9. However, I have a Geissele SSA-E vs his milspec trigger, which I think makes the biggest difference, as well as a higher quality barrel.

 

[1stmarine]

Hi,
the grain of the bullet really doesn't matter but the gas port pressure for the load you are shooting.
Just to give you an example the 55gr FMJ from PMC bronze are up to 250-300fps slower than the M193 from Lake city so while a load might run on the hot side
the other (PMC) might short stroke. In adjustable pistons might need to run the pmc full open while the M193 will run in supressed mode even without a supressor.
So that is why you have the adjustable gas block right? If you feel excesive pressrue you can also investaget other reasons that might contribute to short stroking
like excessive hammer spring pressure, air in the buffer tube or friction somewhere but the difference in pressure between popular rounds of the same caliber
and grain are very real.
Perhaps if you tell me what kind of commercial loads I might have them clocked in my log but a manufacturer might also choose to change their recipe from batch
to batch and year to year. No NATO spec though. That must be consistent.

ps: also the adjustment in gas block might not help someone with a small port. It will help reduce gas block pressure but will not increase the port pressure. Some poeple had to open up the port with mild ammo. typical mistake with M4 style barrels for carbines something that should be avoided in general.

 

[LocoGringo]

I run a Superior Arms adjustable gas block on my 3-gun rifle and tested it just as you described until I achieved reliable lock-back on the action and opened the block 2 more detents for perfect reliability. My brass ejected at about 3:30-4:00 with Federal 55 grain ammo. I broke the new rifle in slowly adjusting the gas block and adjusting the new scope until it was dialed in and tuned up.

After about 40-50 rounds I decided to test the barrel maker's claim that it is a sub-MOA barrel with match ammo and shot HPR 75 grain match ammo and Federal Gold Medal Match 69 grain ammo. While the groups were disappointing, the reliability was absolutely unaffected. It shot perfectly reliably and locked back every time when the magazine was empty.

By the end of the day I guess I had shot somewhere around 60-80 rounds and with perfect reliability after I had made the final adjustments. Perfect reliability with all ammo shot using 55 grain ammo up to 75 grain ammo and no additional adjustments.

I'll wager you'll be just fine. By the way, mine is a 17.3" barrel with rifle length gas, Superior Arms adjustable gas block, Iron City lo-mass bolt carrier and Taccom 10% reduced power recoil spring.

 

[243winxb]

69 gr SMKs (Black Hills) and 77 gr SMKs

The 69 & 77 are in the barrel longer, maintaining the port pressure longer?

A 40 gr bullet would be more of a problem?

I dont have a clue, i just " had to guess"

 

[1stmarine]

The 69 & 77 are in the barrel longer, maintaining the port pressure longer?

A 40 gr bullet would be more of a problem?

I dont have a clue, i just " had to guess"

The grain doesn't matter as soon as one has control over the specific bullets and powders.
However, personally if given the choice of barrel I would stay away from carbine ported barrels and fancy cuts like the M4 style, things that serve no useful purpose and might actually trigger issues. Also people who are seeking maximum accuracy might prefer 5/8 threads or a target crown and stay away from 1/2" threads. IMO those things should have never gone main stream for the civilian shooters.

 

[Doc7]

Mine is an 18" barrel with rifle length gas, 5/8" muzzle thread...

 

[1stmarine]

Mine is an 18" barrel with rifle length gas, 5/8" muzzle thread...

Nice choice. Did you thread it yourself or custom order? It is hard to find 5/8 threads for the 223.
The carbine gas ports can be fine but IMO they are less forgiving and therefore more prone to issues by design than mid or rifle length. Same thing with the fancy cuts of M4 style barrel fashions. Some of the fluting I see is also brutal.

 

[Doc7]

Nice choice. Did you thread it yourself or custom order? It is hard to find 5/8 threads for the 223.
The carbine gas ports can be fine but IMO they are less forgiving and therefore more prone to issues by design than mid or rifle length. Same thing with the fancy cuts of M4 style barrel fashions. Some of the fluting I see is also brutal.

This is the SPR barrel from ARPerformance.