Adjustable AR gas block


July 1, 2007, 09:37 PM
I want to replace a gas block with a gas block with weaver rail on an AR. I saw a JP adjustable gas block on an internet site and was wondering if this is a good product and if there is a benefit from the adjustment capability? Also, which is better steel, or aluminum?

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July 1, 2007, 09:48 PM
Unless you are a competitive shooter, I doubt there would be much benefit from an adjustable gas block...even then I don't see much use for it.


July 1, 2007, 09:54 PM
Are you a man or a woman? *joking* an AR has very little recoil as is. If you screw with the gas system to much you can possibly make the weapon not function. Not to mention you will have to vary your loads you put through the rifle because they will effect the action of the gas system. I suggest just getting a regular gas block unless you intend to use special loads for your AR if you want to reduce the felt recoil put a free float tube on it with DCM match handguards and then add some lead to the front and rear and you will definetly have a change in felt recoil.

The product is good but you have to understand how it works with the rifle.

Steel or aluminum...doesnt really matter its a weight issue.

July 1, 2007, 10:06 PM
Why adjustable? I would go steel, seeing as how the barrel is steel. Too different expansion rates IMO.

Bartholomew Roberts
July 1, 2007, 10:24 PM
Steel or aluminium does matter. Check Zak Smith's report on the 2004 ITRC where aluminium gas blocks shifted as they got hot and caused function issues.

The only big use I can see for an adjustable gas block is to mitigate gas blowback with suppressor use or to tune a competition rifle to get almost zero recoil. It does add another layer of complexity to the system and ARs are very sensitive to having their gas pressures tweaked.

July 1, 2007, 10:32 PM
If your rifle is overgassed (gas port drilled oversize), it could benefit from an adjustable gas block. Turn the gas screw in until the empty cases eject 3-5 feet in the 2 o'clock position.

July 1, 2007, 10:38 PM
I have an adjustable gas system on my FAL and it is handy because I shoot a lot of cheap surplus ammo from South Africa and Argentina. However, it is a pain because every time I strip and re-asemble the rifle I have to reset the gas system. If I am shooting a reliable type of ammo I generally set the gas system and forget it.

In conclusion, if I was building a TEOTWAWKI rifle that would feed any round I could find, an adjustable gas system would be a must. However, on a rifle that only shoots normal and comercial rounds, an adjustable system just isn't needed. So far, I have yet to hear of an AR that won't feed a certain type of 5.56 ammo.

July 1, 2007, 11:13 PM
I would guess the FAL block is a lot more robust than the JP block. Is the JP block set screwed on? If so that's another thing to fail. If this is a fighting rifle you are building than the answer is no friggen way!

July 1, 2007, 11:19 PM
Using an adjustable gas block in conjunction with a muzzle brake can reduce muzzle flip to non-exsistant and increase bolt cycle time.

This can reduce total times in competition and in some events seconds count and any advantage helps.

A properly installed, screw attached, gas block is no more likely to shift position than a properly attached taper pinned gas block.

Steel barrel/steel gas block, aluminum offers no real advantage.

Zak Smith
July 1, 2007, 11:32 PM
The AR-15 "as is" is set up pretty good (at least in mid and rifle-length gas system configurations). Most 3-Gunners want to reduce cycle time and reduce the magnitude of the inertia events to reduce the time it takes to reacquire an adequate sight picture. Slowing the system down generally makes this take longer, not shorter. The adjustable gas block is most used when also using lessor reciprocating mass like one of the lightweight bolt carriers. However, when I had the old all-aluminum JP gas block, I ran the adjustable gas block full open for both reliability and to reduce cycle time.

The expansion rates thing is really only an issue if you get the barrel so hot the stainless is turning colors-- that's the only time I've ever seen it cause problems.

None of my AR-15's now have adjustable gas block-- I run full gas with a properly-sized port.

In AR-10's, which are notoriously over-gassed, the adjustable gas block can be helpful to get the system working better.

With regard to "adding complexity"-- think about what happens when that adjustable set screw gets loose and falls out.

Onmilo, I have direct experience that contradicts at least three things you said:

* practical competition shooters generally tend to want faster cycle time (gun is "in process" a shorter time), not "increase bolt cycle time"

* What does "properly installed mean"? I have witnessed a factory-installed JP aluminum gas block move because of heat expansion and recoil impulse at the 2004 ITRC

* Steel vs. aluminum can make a difference, ibid


July 1, 2007, 11:38 PM
SoCal I am definately a man and am not worried about recoil. I need a good gas block and this thing sounds interesting. Just thought that being able to adjust for different loads might be a good thing.

July 1, 2007, 11:46 PM
I suppose the Real question, Rokman, is what are you going to use this rifle for?

July 1, 2007, 11:51 PM
what zak said. i currently have a jp adjustable gas block on my AR, and i run it full open. next AR i build will not have one. granted, i did enjoy putzing around with it and lots of different loads, since i also shoot a variety of loads with a suppressor. but in the end, i went full open.

in onmillo's defense, zak, i think he did say using the adjustable gas block will decrease "muzzle flip" increase cycle time, which is correct. and it is possible for that to reduce your time on a stage.

there are two times to consider here:
A:time for your bolt to cycle
b:time to regain your sight picture due to recoil

if A is .1 seconds and B is .5 seconds, and you tweak your gas block to make A .15 seconds and B .4 seconds, that's a good thing.

i'm not claiming that's common or in contention with your statement that practical competition shooters generally want faster cycle time. i'm just saying it's possible.

Zak Smith
July 2, 2007, 12:01 AM
If that were the case, yes, however, everyone I know wants the faster cycle time.


July 2, 2007, 12:15 AM
interesting. cycle time has never really been a problem for me. meaning, i've never had to wait on the gun, even double-tapping as fast as i can, it's always reset and fires the 2nd shot. but i can't seem to get my sight picture back nearly as fast as i want.

i need to come hang out with the CO multigun folks a while. :)

Zak Smith
July 2, 2007, 12:17 AM
It's hard to notice unless in comparison to a faster gun. :D

July 2, 2007, 02:10 AM
Seems like an adjustable block might be worth looking at if you were running a really short tube like on a carbine, SBR, or pistol length AR.

Do the aluminum blocks really hold up? I was astonished to see them offered, I figured they would get flame cut, considering how the gas tube get blasted. Anybody ever looked at the port hole on an aluminum block that has seen a bunch of rounds?

July 2, 2007, 09:01 AM
Zak,Uh, I did write 'increase bolt cycle time' and that isn't what I meant.
I did mean decrease bolt cycle time, not increase it.

"Propely installed" means a heat resistant sleeve retaining compound applied, barrel properly dimpled, screws torqued in place and gas block properly aligned to the gas tube port.
Seeing a gas block fail in competition does not contradict anything I said, it just means you saw a gas block fail.
I didn't see this incident and have no comment as to why, or how, that particular block failed.

Read the whole post and you will see I personally would, and do, recommend steel gas blocks over aluminum.

July 2, 2007, 03:13 PM
SoCal I am definately a man and am not worried about recoil. I need a good gas block and this thing sounds interesting. Just thought that being able to adjust for different loads might be a good thing.

Just joking with you Rokman, the adjustable gas block can benefit you but its definetly touchy on an AR if your not careful or sure what you are doing.

July 2, 2007, 03:47 PM
I am new to the black rifle scene and do not like the flip-up sight/gas block combo that is on my rifle. I would just like to put something else on my rifle and ran across this gas block on one of the many internet websites. I am not a competition shooter, just plinker.

Howard Roark
July 2, 2007, 06:06 PM
A lot of highpower shooters are guilty of loading the .223 to it's ragged edge of presssure. Especially the 77 grain and heavier bullets. The bolt cycles so fast it fails to feed. It's nice to be able to regulate gas pressure to the carrier to slow it down.

An adjustable gas tap isn't needed. That problem is solved by moving the gas tap forward a little to cover up part of the hole in the barrel. When the brass is ejected to the side, not forward this problem is solved. I've done this on rifles shooting as light as 69 grain bullets.

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