Best "long term" adjustable gas block?

AR/M4 AR Technical Discussion
61

Physics tells us that there’s no free lunch. If you decrease the amount that goes into the gas tube the rest must go somewhere. The real issue is how much gas the port lets out if the bore. If it lets out 20% more than is necessary to operate the action then the block has to vent the other say 18% that is not required to run the action.

The BRT solution is exactly the same as an AGB except it permanent and doesn’t have the adjustment issue. If you were to disassemble one of those, you would find it very dirty and possible seized into its position. It does provide a solution to the over gassed rifle but it’s a single size solution. I’ve used them, they’re great for FSBs that have no adjustment option.

The problem here is how much gas the port allows to leave the bore. If you tune it down, you’ll have to deal with where the excess will vent.

62

The adjustable gas block for the AR does not have to vent gas at all. It only has to restrict gas flow. The rest of the gas is vented through the muzzle.

If we were talking about the FAL gas regulator, that would be another story. The FAL regulator works by controlling how much gas it vents at the block, not by restriction

63

This is kind of what I was thinking. So, it is just a matter of the path of least resistance when the excess pressure to blows down and the inherent design obstacle of an adjustable regulation mechanism using the threads of a screw susceptible to seizure, I gather?

64

You are correct but there’s no free lunch. It does restrict the amount of gas allowed into the gas tube but what happens to the rest of the gas that leaves the gas port? The pressure gradient is too high for it to be forced back into the bore. Where is it supposed to go? If the restriction only solved the problem then the adjustment mechanisms wouldn’t become fouled and seize up in certain cases.

This would also not explain why my (and others’) left hand was black from gas escaping THROUGH the Syrac’s adjustment mechanism on the left side of the block. Consistent with this venting, the SLR vents forward around the adjustment screw as mentioned.

The gas that does not escape the gas port is vented out the muzzle when bullet uncorks it but until then the pressure gradient between the gas tube at the gas block and the bore at the gas port is too great for reverse flow; it’s a one way street. I think I read on AR-15 barrels that the port pressure on a 10.5" carbine gassed barrel is like 24,000 psi. I know that’s only until the barrel is uncorked but that’s still way too high to be anything but a one way street.

The way I see it is that you are correct in that the AGB is designed to restrict and not vent as foes the FAL block BUT the AGB must take into account that there needs to be a less restrictive path for the extra gas from the gas port that is bled off and not allowed down the gas tube. One thing we all forget occasionally (myself included) is that the gun is over gassed or there would be little need for an AGB (save for fine tuning suppressed guns).

I’m not a physicist but I can’t construct any other reality on this one based on my experience with fouling and seizures; the path of the fouling is objective and consistent and doesn’t suggest any other conclusion to me.

I’ve tried to provide the best info I can to help the collective here (Gid only knows how much y’all have helped me).

Good luck!

65

But theoretically speaking if it were possible for the adjustment mechanism to be sealed while restricting gas flow it would simply hold the pressure until the barrel pressure blew down enough to equalize and expell it out of the muzzle right? I fail to see how gas block insert regulation does not function this way with the exception of the seal between the barrel and gas block.

66

Theoretically, I think you’re correct. The system would certainly have to be very tight. Honestly, I’m not even sure the block to barrel fit would be a problem as long as the block is not oversized. Once the bore is uncorked there would be very little to no pressure gradient so I would think said gas would just stop moving and not be an issue.

67

This is part of the reason behind KACs new design using a type of castle nut to sandwich the gas block between the nut and the barrel gas block journal shoulder. Also, they use a different fitting to attach the gas tube to the gas block proper. It’s basically as sealed as one can get.

68

Ooooh! I’ve not seen that. You really shouldn’t have told me that. Lol. Thanks buddy! Now I’ll have to check that out.

69

I’m here to help (you spend more money). :wink:

70

Some observers after experimenting with carrier-key regulation have noticed excess gas from the pressurized gas-tube blowing out every which way it can from around the gas-tube hole/roll-pin and between the barrel and gas-block/front sight base more so than if pressure were not captured in the gas-tube.

So getting back to adjustable gas-block design limitations, in summary, the inherent venting of gas out of adjustment screw designs and the ensuing carbon fouling is the main reliability issue to contend with after designing a solid detent mechanism. Potential resulting issues include:
1.) seizure of the adjustment screw rendering the adjustablility of the GB useless and thus affecting reliability depending on the setting it seized at, the rifle’s set-up (dwell time, gas port size, buffer weight, recoil spring, hammer spring, bcg, muzzle device back pressure, and suppressor), ammo used (556 vs .223), and firing conditions (air temp, dirty gun, etc.). This may not be an issue if the user’s intention was to set it and forget it at a setting allowing a good margin of function (but then perhaps a gas-block insert like BRT offers is the best bet, imo)

2.) The spring detent mechanism may seize due to carbon fouling either resulting in the screw no longer moving which thus affects reliability as outlined in #1, or it may seize open which would render the detent useless and allow the adjustment screw to spin freely and even fall out, leaving the rifle deadlined.

It has been mentioned the spring detent design of the Syrac is susceptible to seizure as outlined in #2 because it is exposed to venting gas and the internal spring can be fouled inside it’s hole. After messing with the the SLR Rifleworks design there is doubt the external leaf spring detent design is subject to this type of failure. It seems fatuige is the only thing it could be susceptible to but it is very stout and rated to >2000 degrees.

That gas will vent out of the adjustment screw is inevitable with the potential to seize the threads, all current adj gb using a screw are susceptible to failure #1 but SLRR has attempted to mitigate the effects of fouling by using looser fit thread tolerances that better accomodate the venting gas by creating more room for fouling in the threads to accumulate while still allowing for the screw to move.

It seems clear the best current adjustable gas-block design is the SLR Rifleworks Sentry series so long as one tends to the threads of the screw with Fireclean and by breaking the fouling free by twisting the screw back and forth every few hundred rounds. But is this suitable for a fighting rifle to avoid failure #1?
I think so, assuming a quality rail shrouds the GB, since likely the gas-block setting on a fighting rifle will already be set for an appropriate margine of function. However, for ultimate reliability under a “set it and forget it” mentality, a gas-block insert suitable to ones barrel length, gas system, and rifle set-up may be the best way to go. Albeit the insert requires more research as to which port size insert is best for the set up and more commitment upon installation.

My question for Clint is, will the gas block insert being made of stainless steel (Tom mentioned inconel) outlast a barrel in regards to erosion?

71

While I’m certainly biased, the SLR is the superior extant design because the spring is unlikely to become seized in the open position. Unless it were fired for many hundreds of rounds in between detents, not enough fouling could get under the spring to allow the screw to spin freely.

Again, a drop of FireClean from a needle oiler every 400 rounds or so is cheap insurance.

Besides, after 400 rounds with a can, there are other PM issues that need to be addressed on the weapon system as well. Like lubing the shit out of the BCG/CH/Bolt and the rest of the upper.

In terms of being “for a fighting rifle”, one would do well to not change their weapon setup mid fight, or maybe they should set the AGB such that their gun will still run sans can if they need to remove it for some reason (I.e. it gets damaged, crushed, etc). Or, have a secondary weapon system to which one can transition.

Context is important in this discussion.

72

Good points gentlemen. My kudos to Jerry to putting in a nutshell what I can’t put in a barrel without a bottom. Lol.

73

Of course, not adding an adjustable gas block eliminates any and all problems associated with adjustable gas blocks…

74

Agreed, dedicated porting is the better solution. Another less preferable possible solution to an over gassed example could be an insert in the gas block, given that the system doesn’t leak excessively.

75

Just to note, PWS has had a castle nut securing the gas block longer than Knight’s. It launched with the MOD1 series. When I got my SR-15 MOD2 upper I said, “Cool, it has the same gas block securing method as my PWS MK2.”

Also, the gas tube to block fitting on the MOD 2 is derived from liquid/gas fittings. I have a bunch of Swagelok ones at work.

76

Agreed, but how does one do this? It seems most manufactures lack the stones to put their name on something that might not function with Tula .223…

Also where did you get an inconel GB insert?

77

You’re over-thinking it. The gas simply follow the path of least resistance. If that path takes it from the gas port and through the restriction created by the adjustable gas block and down the gas tube, that’s what it does. if the pressure in the bore drops enough, the gas will vent back into the bore.Gas flows through an adjustable block the same as it runs through a standard block.

The reason gas blows through the adjustment screw, out the gas tube roll pin and what have you is because are leak paths. It’s higher pressure gas following the path of least resistance. If the gas block were perfectly sealed, the gas would’t leak out at those points. And, as the AR adjustable gas block is restricting gas flow, there is no excess gas.

When the bullet uncorks the muzzle, the pressure does not just drop to nothing. It takes time for that gas to escape and vent. In fact, the gas system doesn’t pressurize until after the bullet has exited. The gas system runs off of residual pressure

78

Read this thread all the way through https://www.m4carbine.net/showthread.php?33743-Best-Suppressed-SBR-Ever

79

I am familiar with this thread but Grant seems to have gotten a special order barrel from Noveske with a small gas port. I know SIONICS is testing “reduced” gas-port barrels for mixed suppressor use. Other than that the trend is for gapping gas ports, BCM and SIONICS current barrels are on the more conservative side of the industry, though.

80

Grant opened up the gas port a little at a time until he got the AR cycling the way he wanted. You will also note that the LMT Enhanced carrier proved to be crucial