I'm going to try this out next weekend.  Just asking if this is a somewhat correct way of thinking, before I potentially waste time.

Standard 556 ammo, say Lake City green tip / XM855.

Tight suppressor with normal to higher gas blowback, I am using the Griffin GP5.

Using an adjustable gas block, LOW mass carrier and H1 buffer and Sprinco white / normal or Sprinco light / yellow spring.

Whatever gas charging handle, for discussion the Gas Buster handle.

In theory, would that system use the least amount of gas to cycle, thus directing the gas down the barrel more?

Or am I using too many lightened products that will make the system less robust?

Boy you've done it now...

Yes, less mass will require less gas. Also leaves still shallower margins to overcome fowling. That lower mass still has to return to battery. I think you are fine to run that way, and as I'm not familiar with lightweight setups I will stop here save to say I agree with the poster below. That gas has to go somewhere and it will find the shortest way there, be it out the receiver or down the barrel.

The location of the gas port is the number one factor IMHO.  The farther down the barrel the gas port, the more the chamber pressure will fall before the bolt unlocks and the more gas that will exit the muzzle instead of following the brass out of the chamber and into your face.  I've never felt the need for a special charging handle on my 18" rifle gas suppressor host AR.

I know it, that is why I put this in the silencer sub, I have confidence it will not degenerate.

This specific rifle I am building, which is exactly what you said.  Exactly my goal.

1.  Using a Griffin GP5, thread on can.  I plan on welding it on.  No removing it.
2.  I plan on using a very narrow supply of ammo.  Bulk M193 or XM855.  Remember I still have the adjustable gas block if needed.

One stamp gun, done and done.  Right now it has a brace on it.  I absolutely do not care to discuss brace vs stock.

Lol nor do I. I amended my response about 6 times because I'm on 3 hours sleep and have no business reading to typing at the moment. I think we summed up the gist of it.

Gotcha.

This upper has a carbine gas system.  11.5 inch FN chrome lined 1:7 barrel, made by FN for PSA.  Its a fine barrel.

Apologies, I should have put this in the original post, as it is pertinent for this discussion.

Edit:  I'll try it out, if there are no major objections on reliability or longevity of this idea.  I do know the further I get "out of spec" the potentially worse its going to be.  How far am I going out?

I tried a number of testing configurations on my 14.5 midlength and found that adding mass to the carrier didn't have a particularly noticeable effect.  Neither did swapping between a BCM gunfighter charging handle and a Geissele SCH. An AGB helped some, a LMT enhanced carrier helped more, and a Tubbs full strength spring the most.  My theory is that the LMT enhanced carrier with a spring that is similar in strength to a full sized rifle spring both delayed the unlocking and allowed more time for the excess gas to vent through the extra holes in the LMT carrier.  It worked so well I cut down another Tubbs spring for my pdw 300blk and it shoots much more smoothly now.

The 14.5 gun went from downright unpleasant to just fine.  There's still some gas coming from the upper, but I can shoot fast without tearing up now.

I recently bought a Griffin AR-SOB buffer, but have not tried it.  It is between h2 and h3 and has some minor tweaks over regular buffers?

Thanks for saving me frustration.  Honestly.

I’ll keep with full mass carrier and h2/h3 buffer.  White spring, which is standard.

I also have an extra Kynshot 556 I may trial.

My Faxon 11.5” mid with the giant gas port runs great with an SLR-6 AGB and a relatively high back pressure can (.30 Omega).  However the front gas adjustment is practically right up against the ASR muzzle device. I had to grind down the short leg on an Allen key to be able to adjust it.  A side adjusting gas block you run into access issues if the handguard covers the gas block.   For a pin and weld, handguard selection gets more critical and limited if you want to be able to slide a free float HG off and over the can.

I recommend getting everything checked out and running flawless before welding permanent.

That is a given.  I understand proper preparation.

I mean, if it isn’t (within norms) accurate, I’m aborting it.  Even if it functions perfectly, even if it handles perfectly.

The adjustable gas is from Ecco, on a standard front sight gas block.  It adjusts from the side.

That’s one of those ideas where you see it and wonder why no one thought of it sooner.

And in that configuration the handguard concerns are also no issue.  I bet it shoots fine. If the HPA had passed I’d have a few like that sooner rather than later.

Yea.  With the suppressor on, I wanted to have lighter hand guards.   Really, the purpose of these hand guards, are to protect my hands.

I went with Magpul SL since they are light weight and have mlok mounts if needed.

I drilled a hole in the plastic to adjust the screw.  Easy and effective.

Before I start throwing around a bunch of stuff that sounds authoritative - I'm no SME, I'm just a mechanical engineer who has been trying to suss out the variables in my free time. Most of this is theoretical, and I'd defer to an experienced gunsmith or armorer.

I think the gas in the face phenomenon is a timing issue for ARs. To understand this, let's think about pressure at three locations: the chamber, the gas port, and the muzzle (bare or inside the suppressor).

Let's start with the unsuppressed case. After the bullet passes the gas block, but before it passes the muzzle, the pressure at the chamber and the gas port are similar. The muzzle is at atmospheric pressure The gas system drives the bolt back to cycle it, but the bcg has not made any appreciable movement rearwards by the time the bullet has left the muzzle. Important to note, the muzzle is still at atmospheric pressure (more or less) as the gas exits, meaning there is very little resistance to flow and the pressure drops quickly. Gas is currently flowing out of both the gas tube into the receiver and out of the muzzle after the bullet exits. This allows much of the pressure at the chamber and at the gas port to dissipate before either are opened and are a path for gasses to travel. The residual pressure vents out of all three openings as the bcg cycles and chambers a new round.

Now the suppressed case. The difference occurs after the bullet leaves the muzzle and gas begins to fill the suppressor. This means the pressure at the muzzle is not atmospheric pressure, but is the "backpressure" of the suppressor and this "backpressure" decays slowly relative to the pressure decay in the previous case. This backpressure inhibits flow from the chamber and keeps pressures in the entire system elevated after the bullet has left the muzzle. The pressures are still elevated as the BCG moves rearward and begins to extract the spent round, resulting in both increased friction in the chamber (due to the brass being under pressure) and a sudden opening of a "path of least resistance" causes trapped gasses to flow to the chamber (which is exhausting at atmospheric pressure now, not the elevated pressures in the suppressor.) It wouldn't surprise me if in some circumstances gasses trapped inside the suppressor actually exit through the chamber and not through the muzzle of the suppressor.

The peak pressures in both systems are the same, it is the decay in pressure that differs. So, to minimize gas blowback (and action noise, and improve reliability) the key factor is to reduce the chamber pressure at extraction. There's three main ways to do this.

Firstly, if you extend the barrel and gas system, the extra barrel length already decreases the pressure. I attached a graph below from this excellent report on that subject: Barrel length vs pressure The lower pressure starts you off in a better spot regardless of other mitigations. Of course, if your goal is to build a SBR, me telling you to use a longer barrel doesn't help anything.

Attached File


Secondly, you can change the timing relationships. The idea here being to reduce the time between the bullet passing the gas port and the bullet exiting the barrel (dwell time) while delaying the BCG moving backwards to begin extraction. Altering dwell time has a lot of potential to interfere with proper function of the rifle while unsuppressed. It's a much more complicated problem to optimize a rifle for performance both with and without a suppressor. If you're happy with suppressed only, it's worth looking at 16"/rifle length, 11.5"/midlength type options. Beyond that, you want to slow down extraction. This is where increasing BCG/buffer weight and reducing gas flow (smaller port, adjustable gas block). One very interesting product for that is the LMT enhanced BCG. I noticed a couple people above said they'd had good luck with them (I'm still waiting on a backorder...we'll see if they beat the ATF or not). They changed the cam path for the BCG, which delays unlocking.

Finally, you can change the backpressure of the suppressor. This is the approach the OSS suppressors take and while I've never tried their products, it's an interesting approach. Beyond that, shorter fatter cans will be better for backpressure and worse for noise levels.

Personally, I've been toying with the idea of a pin and weld to 16" cut down of a rifle length gas barrel, coupled with a VLTOR a5 buffer setup and an LMT eBCG to try to make a suppressor centric AR.

@katanasword

Thank you.

Reducing mass and using an AGB to turn down the gas to reduce recoil and muzzle movement in the AR is standard practice for competitors.  But if you try to do it with typical suppressors, you will get MORE gas to the face.  This is because the chamber will unlock earlier and faster with less mass, even if you turn down the gas more.  Turning down the gas doesn't make up enough ground for the loss of mass when dealing with high back pressure suppressors.  You want to increase the amount of time the BCG stays locked as much as possible, and to do so I'd suggest an LMT Enhanced BCG, Vltor A5h2, and a superlative arms bleed off gas block.  That combo gives the best gas in face and noise to shooter results out there.

That is a very good explanation.  I know this is a bit nit-picky, but the gas system doesn't drive to bolt back to cycle it.  The gas system drives the carrier to the rear which unlocks the bolt and allows the pressure in the chamber to cycle the action.

ETA-. I believe a 16" with rifle gas system is an ideal setup for a suppressed AR and a 14.5-14.7 with rifle gas may well be ideal for a dedicated suppressor host.  Another thing that can help is to use a 30 caliber suppressor which is often just as quiet as a 22 caliber can due to larger volume, but allows more gas to go out the muzzle rather than back down the bore.

I took the advice and this inexpensive build ramped up quick.

I added an Armageddon charging handle (like the gas buster), adjustable Ecco gas block modification, and (most expensive part) Surefire optimized BCG.

This is a pistol upper because the suppressor isn’t permanently attached.

I timed the adjustable gas to a pistol lower with a VTLOR system, the one in the back ground with the SB Tactical brace.

Gas to face was much much reduced.

I then loaded it with some Wolf steel cased, which I’ve noticed is more gas prone.

Ran it in auto, on the RR and it’s far far less gas to the face.

Thanks for the advice.

Attached File

For semi-auto the SureFire OBC is probably cost-overkill for gas mitigation, but in full auto it eliminates bolt bounce and slower ROF gives time for suppressor dirtied mags to lift cartridges and feed.

What did you think of the recoil impulse and lowered firing rate?

The Griffin GP5 is not a lightweight suppressor.  That front weight with the Surefire BCG, it was quite controllable.

I’ll most likely pull this Surefire BCG out for semi, and then use it in auto.  Or for the COVID apocalypse.

Love my faxon mid 11.5, in fact, just ordered another! Cycles low power .223 (PMC) great unsupressed, and runs smoother than anything I've shot when suppressed (sandman S) with the bootleg BCG turned down one notch.