Adjustable Gas block?

[Lowrider]

Noveske switchblock in a slightly modified Troy MRF. 

[Lowrider]

Noveske switchblock in a slightly modified Troy MRF. 

a closer view

[EasyEJL]

hmm yeah, mine is a low profile, so I don't need to machine away the top, and it is an adjustment screw directly on the left side. it just lines up between the ventilation holes in the mrf. i'm going to go take a pic

[EasyEJL]

my cellphone's flash is sucking. Anyhow you can see the gas tube roll pin hole in this pic, but not the adjuster screw

 

 

and here's the adjuster, basically right in the middle of the 2 vent holes

 

[Lowrider]

What brand is that block?

[EasyEJL]

its a BTE, simple and inexpensive

[Lowrider]

nice looking block. 

[Ben Diss]

I can't reach the screw on my adjustable block with the troy MRF rails installed. ugh, that will be fun to tune.

 

Syrac is adjustable from the front.

 

[StainTrain]

Syrac Ordinance is by far the best I have used.

[pridekills]

I can't reach the screw on my adjustable block with the troy MRF rails installed. ugh, that will be fun to tune.

Take the rail off while adjusting the gas block.  Get it set then reinstall the rail.  I used an Apex handguard on my build which made it pretty easy.

[Justintoxicated]

I put a syrac on my AR15 it works great, I like to not have to use locktight.  I swear that once adjusted my brass was coming out at the 4-5 o'clock position though, but it certainly seems to screw the brass up less, and not send it flying away nearly as hard.

 

For my AR though the barrel is .936 diameter so the syrac isn't even close.  I don't know why they don't make them in more sizes :(

 

I also wanted the adjustment on the front so that I would not have to screw with my 15.5" handguard that covers it up.  Wilson Combat had just the thing.  Very similar quality to syrac in machining, materials and finish, but I will have to use locktight once I figure it out (I'm not even sure there would be room for a detente the tolerances are damn tight (maybe that's why syrac hasn't come up with one yet?).  It isn't the preferred clamp on style, but machined well enough that I'm hoping to not have any issues with the bolt on kind.  It just BARLEY clears the JP handguard, I don't think a clamp style would work for this anyways because they are typically larger.

[.308LiteHunter]

Anybody used a railed adjustable gas block, or just the low profiles?

[Powerman]

guys,

as I understand it, its not a PSI thing its volume thing. As we know with any Ar system (15-10) the goal is to get the brass to eject at the 2 o'clock location. This shows a system that is timed. As you increase the volume of gas, the brass fly's to the 3 o'clock, then to the 4 o'clock and recoil and wear on parts in very high.

as you get to the 3:30 and 4 o'clock eject locations you will see after 30rnds some will stick and not eject, or rip off the base of the round.

So if I get a load, lets say 168 FGMM and time it to a 2 o'clock position, and then shoot 175 SMK FGMM the adjustment of the timing is very little. As a matter of fact I have found that 155grn, 168, 175's loaded to a normal level, the brass is with in 3" of each other in piles.

So that tells me the timing is good for two rounds, and great for one of them. This also tells me the volume of gas is not that different between the three.

now with timing, the main issue is dwell time also. This done by the gas port size, and the amount of barrel after the gas port to the crown of the barrel.

Case in point.

The dwell time on a 20" and 14.5" barrel is the same. 7" gas system and a 12" gas system. 8" this is why the two barrel systems where used back in the day, to keep the same dwell time between ammunition.

Now the amount of gas between the cartridge and the gas port is not the same. So this had to be adjusted by the gas port size. So what is a manufacture to do? drill the smallest hole for the widest amount of loads, this is also part of why heaver bullets shoot like crap is the wrong twist rate. The timing is off due to the slower bullet speed, hence less gas pressure.

Now one you get the smallest, hole drilled for a wide range of bullets and powder loads, this works but can be hell of the parts, as we all know. If you adjust from a full ON level to a manageable level that lets the bolt travel to the rear of the magazine, but not slap like hell on the buffer and compress the soft plastic stuff, you are saving parts and lowering felt recoil.

This is why I feel a adjustable gas system is needed on 308 AR rifles. try it, its only $50 and if it does not work, turn it all the way on and be done with it. But you will see a big diff in the performance of the rifle.

John

 

I realize this is an old thread, but for anyone reading this... this is not correct. It most certainly is a PSI thing. Volume is fixed.

 

The gas acts on the action like a piston and cylinder. "X" amount of surface area of the piston, acted on by an energy with so many pounds per square inch, transfers "X" amount of energy to the bolt carrier and moves it. The only difference is the cylinder moves in this case and not the piston. It does that in the space needed for the carrier to move back far enough to cycle the bolt out of battery and free from the barrel and move it back past the gas tube/gas key which then allows all the pressure to exhaust out of the ports in the carrier and gas tube. Of course the bullet exiting the barrel helps. :)

 

But the bolt and carrier have so much energy to cycle back, pick up a round, and slam into battery. The only thing stopping it is the spring when it fully compresses and bottoms out against your butt stock and then your shoulder. But springs do not supress energy, they store it... which is then returned in full to the bolt and cycles it back into battery.

 

Now I'm no gun expert, but pressure changes caused by more or less powder will most certainly change all that. However, I do not know how much of a pressure change from the powder is needed to make a significant change in operation of the gas system and what that means to recoil, or not enough energy to cycle the action. But just going from 7.62 to .308 changes powder charge.

 

Volume is fixed. It is the space from the bolt face, to the rear end of the bullet, when it passes the gas port, and high pressure gas pressurizes the gas tube all the way to the chamber formed by the carrier, bolt, and gas rings. That volume never changes. It will be constant no matter what you shoot. What does change is how much pressure is in that volume caused by how much powder was used in the particular load.

 

Now all of that does not change the fact that if a rifle is over gassed, you will feel more recoil. You only need enough gas to fully compress the spring past the bolt catch. Any more than that, the spring will "bottom out" angainst your shoulder and you will absorb the remaining pressure that was unneeded. Once the spring is fully compresssed, the gas has nothing to do with it any more and it is the full spring pressure that returns the BCG back into battery. Not enough compression, not a proper cycle.

 

So the spring has to be strong enough to pick up a cartridge and return the BCG to battery. The gas has to be enough to fully compress the spring. More gas than what is needed to compress the spring, will be felt as added recoil to the shooter. Not enough gas pressure to compress the spring, short cycle... no lock open, round not picked up, or bolt jams on the case.

[Justintoxicated]

I ended up with a Wilson Combat adjustable gas block.  I like my syrac clamp on better, but the wilson is the same quality and adjustable from the front for a .936 diameter gas area.  It's not clamp on though because then it wouldn't fit under the FF tube, I also like the detent on the syrac, but the Wilson does have a brass set screw to lock it in place so no locktight is needed.

Edited May 20, 2013 by Justintoxicated

[texas30cal]

I am going to drill and tap my current gas block does anyone know what size and pitch most of the maufacturers use for the adjustment screw?

[texas30cal]

Well I didn't get any replies so I just went ahead and did it, tapped it from the side instead of the front and used a #10-32, not sure when I can go to the range but will report back.

[Uncle PoPo]

I bought a Syriac and am having trouble with setting it. The screw is all the way in, and it still cycles. I would think with the screw all the way in there wouldn't be any gas to blow back the carrier. I tried emailing Syriac, but still haven't got a responds. Any ideas on a fix?

[edgecrusher]

Refresh our memory on your firearm

[EasyEJL]

And what specific ammo you are using

[texas30cal]

I still haven't made it to the range, it's hotter than the surface of the sun here now, maybe I will go early one morning, I'm too damned fat for this climate!

[Rsquared]

If the adjustment screw is all the way in, than yes...the flow of gas should be shut off at that point. Syriac makes some higher priced stuff. You would think that you wouldn't get a defect. But that doesn't mean that it doesn't happen. I've been planning to try some of their stuff on one of my next builds.

[texas30cal]

Could be a shaving that didn't get blown out of it in manufacture, or maybe the hole didn't get tapped deep enough, both easy fixes.

[Jgun]

Anyone tried this one? A bit pricey but it might be worth testing.

http://micromoa.com/

[Robocop1051]

Anyone tried this one? A bit pricey but it might be worth testing.

http://micromoa.com/

 

That sounds like you're volunteering  :banana:

[Rsquared]

^^^^ I was thinking the same thing.