Influence on operation of rifle concerning muzzle devices.

[jaxman7]

This is not a thread concerning muzzle movement characteristics with x brand muzzle device vs. y brand.

We all know cans add a great deal more pressure to the gas system as compared to a rifle that’s unsuppressed.

Cans are completely enclosed except for the bullet tunnel.

Looking at different MDs it made me wonder if (with a rifle at the edge of reliably functioning/smooth shooting…Buford T’s service rifle comes to mind) some devices create more backpressure/slightly increase dwell time.

I have 4 at the house. A BCM comp, a BC, a FSC556, and AAC’s Blackout.

Looking at the AAC there is hardly anything restricting the gas and slowing the expansion rate down from venting out. Now take a look at the BC. Compared to the Blackout it is much more of a ‘captured system’.

This is all just straight conjecture obviously but it would make sense a more restrictive type compensator would theoretically lean more towards performing in the same manner as a can than say a 3 open pronged FH. Question is, is there enough of a difference to alter function?

It just got me thinking after rereading Buford T’s thread on his rifle.

(https://www.m4carbine.net/showthread.php?t=109353)

He stated during one test that it was running on the very edge of reliably functioning. He wasn’t talking about MDs but it started a line of thinking in me as to what else not mentioned in his thread could contribute to the fine line of a gun running great/smooth and one not running.

I am about to start building my own version of a super smooth shooter soon (have all the parts just need access to a certain machine) and am looking forward to the results.

Anyone run into an issue with function variances with different muzzle devices?
I am not concerned this will happen with my new build but its a subject of curiosity.

-Jax

[Blowby]

I have all the comps except for the BCM that you listed. The AR’s exhibit no loss in reliability with the MD’s. I had the same thought when building the Noveske 16" to reach out a little. I chose the AAC for the reason to minimize influence on tragectory. Not sure if this made a difference but based on you posting similar concerns my choice has allowed me to hit out to 650-700 yards on a steel man target with 80% success with the limited long distance skills and the scope I had available.

Edit: After reading the linked post above, after the fact, my post is not on target for the extensive gas port/BCG/Spring combinations you are discussing in the common topics of these threads.

[BufordTJustice]

Jax, great post brother.

I know that some muzzle devices (Like the Noveske KX3) can have an effect on overall gas system pressure. However, the KX3 is virtually sealed and is designed to contain pressure inside of its body and the only hole is, as a suppressor, the bullet exit aperture.

I’d imagine that some cyclic rate testing would be in order to test your hypothesis. I’ll leave it up to smarter minds than me to elaborate on gas system pressure effects. I know that the std wisdom is that gas system pressure drops to zero as soon as the base of the bullet clears the muzzle. I also know that my carbine setup that you referenced was not originally “supposed to work” either. But it did…and it’s still working. Goes ot work every day on patrol with me, in fact.

[jaxman7]

Thanks for the feedback Blowby and Buford. I don’t have near enough the knowledge or equipment to test this out but maybe someone has.

The pressure should drop to zero upon bullet exit but something like a KAC Triple Tap I would think delay that ever so slightly. Like I said it’s probably so minimal any influence on function is irrelevant. Interesting to me nonetheless.

-Jax

[Brahmzy]

Essentially what causes the can to change the gas behavior is the sheer increase in length of travel / time before the projectile leaves the weapon. Once the bullet has exited the crown, gas will flow around the bullet and out past the front of it with the little clearance it has inside that can, but as far as cycle-pressure is concerned, you might as well have added another 6-8" of bore onto the gun.
Just imagine a 20" barrel with pistol-length gas and the violent behavior that would create.
That said, I think it could depend on the MD’s length, how much it is vented or not vented, just as you were saying. Maybe I’m just repeating the OP. I suppose an SJC Titan or SF brake would be on the open extreme while a can would be on the closed extreme. Since a KAC TT is so short and the time the gas gets to vent (even through those tiny slits) is immediate, I don’t see it having much of an affect at all. I think it all has to do with time, which means length of said MD. What about the Colt Commandos? Anybody have one? Aren’t those a 10.5 barrel with a 5.5" FH? Pistol gas? Or no, carbine gas.
Shit I don’t know. I think most short MDs would be insignificant on cycling behavior.

[jerrysimons]

@mtdawg169

There’s a very good article in the new issue of Combat Arms on the new KMR uppers. It included a picture of the stripped ELW barrels as well. It stated a 0.625 oz. weight savings on a 14.5 barrel. An interesting note, it also discussed the Mod1 comp and described it as having an expansion chamber that improved reliability of the 14.5 midlength barrel. I had not heard that before. Overall, it’s a good article. The issue had some of the best written articles I’ve seen in a print magazine in a long time.

@ESK

You are correct. Any enclosed chamber is in some respects an expansion chamber. Some comps and/or flashhiders are better then others. The Mod1 has a very small chamber, but it still tricks the weapon into thinking it has an inch longer barrel (after the gas port) than it does… Any increased dwell time is a benefit, especially for a 14.5" middy.

@ESK

Just the Mod1 and the 762 Comp.

@uffdaphil

That was my logic for using a Troy Claymore on my 10.5". After reading the BCM thread stating a 40% dwell increase for 11.5", I figured mine should gain a significant amount. Just a guess on my part as I understand the physics involved about as well as I do women.

@BufordTJustice

On a related, yet still-off-topic note, I wonder if this is part of the reason that many brakes like the BC, AFAB mini, GA Flash Comp II, MAMS, etc. all have been reported as having a sharper recoil impulse? Possibly due to increased rearward BCG velocity from the added dwell time and gas volume afforded by what is apparently tantamount to an expansion chamber in each of these devices?

@ESK

Yep, you are correct. Some of those mentioned comps have excessive expansion chambers, which trick the weapon into thinking it has a 2", 3" and 4+" longer barrel after the gas port. That’s the beauty about the AR platform, at least we can balance it out by adding more reciprocating mass, i.e. the A5 buffer system.

@BufordTJustice

Thank you for replying. That’s very interesting.

How do you know what constitutes a properly sized expansion chamber for a given barrel length, gas port location/size, and ammo pressure range?

I’d assume that the size of the chamber (diameter and length), the shape, and the number/surface area of the exit holes would play a role in this calculation?

@ESK

As I see it, you kinda have to work around what’s offered to you, meaning the barrels are a set specifications from the manufacturer. It’s not to often we can have a barrel made to our specifications. So, I usually work with what I have. There are many of time I’ve chopped barrels down, because A) the gas port is too big and B) there’s too much barrel after the gas port. I only install a expansion chamber type Comp/FH when the barrel is to short after the gas port. I like to run just FHs or non-expansion chambers Comps when running a proper length barrel. My biggest pet peeve with ventilated expansion chambers is the occasional fireball they produce every few rounds. I like my flash (or lack there of) as consistent as possible. The rest of the formula is working on adding or subtracting reciprocating mass, keeping it balanced around ammunition types.

@jerrysimons

Every once in a while a post blows your mind…
Could this be a mystery variable as to why some barrel-length/gas-system/buffer-weight setups seem to defy the conventions of what should cycle?

Which devices have expansion chambers and which do not? How do we quantify them?
As much as we obsess about buffers, gas length, barrel length, gas ports and even LMT enhanced carriers, how have we over looked this aspect of “tuning”?

From the KMR thread: https://www.m4carbine.net/showthread.php?135121-New-BCM-rail/page112
Bump for discussion in the appropriate thread.

[bruin]

Could this be the ticket for 16" rifle gas?

[MistWolf]

@jaxman7

…The pressure should drop to zero upon bullet exit…

Negative. It takes time for the pressure in the rifle to drop to atmospheric after the bullet uncorks the muzzle. Good thing too, because it’s that residual pressure left in the system that allows the rifle to function

@Brahmzy

Essentially what causes the can to change the gas behavior is the sheer increase in length of travel / time before the projectile leaves the weapon…

Not quite. What a suppressor does is increase the time it takes the gasses to vent the system after the bullet uncorks the suppressor. The suppressor is a pressure accumulator which slows the gas velocity and, in this case, also cools the gasses. Slowing and cooling the velocity of the gasses before they exit increases the time it takes for the system to blow down to atmospheric and is what reduces the sound energy

@bruin

Could this be the ticket for 16" rifle gas?

Yes. Colt first used a moderator to increase blow down time so the “CAR-15” would function reliably

[jerrysimons]

So let me get this straight, how do you know if the brake/comp has an expansion chamber? Are there different types of expansion chambers?
1.) Are we talking an enclosed internal portion of the MD just after the threads with some sort of bulk head before the baffles/main chamber where the gasses are redirected/vented?
2.) Or are we talking the chamber from which the ports drilled on the top of some MDs vent the gasses upward before the baffle section/vent chamber?
3.) Or in the case of the Battle Comp (and similar), is the single baffled vent chamber also functioning as an expansion chamber as described, or in addition to #1?

After identifying what the expansion chamber is on various comps how do we go about figuring out which ones have the greatest effect on dwell time/port pressure. This info could be advantageous in selecting parts for both under gassed guns and over gassed guns, or when tuning at the ragged edge. This seems to be an important variable to consider and perhaps is a reason why it is so hard to even ball park general impressions of a comps/brakes effectiveness for mitigating recoil/muzzle rise. In effect, on certain guns, the comp could be working against itself. On one hand it increases overall pressure (ESK cited +4", woa!) while on the other hand it redirects gas forces to mitigate the recoil.

As complicated as these variables are, especially as they overlap, the more I am convinced of using adjustable gas blocks (go to the source). Why do we stretch the gas system to mid-length and the chop the barrel 14.5" only to then add a comp that adds 1" more dwell time for reliable function when we could have just had a 14.5" carbine length gas system?

[Clint]

Just to reinforce this point, the bore pressure does NOT drop to zero as soon as the projectile clears the muzzle.

This “blowdown time” is an exponential drop off that lasts about 3ms in an AR.

It varies with muzzle pressure, weight of propellant and bore area.

The falling pressure during blowdown time makes up roughly half the total gas drive in a DI AR.

The old “gas trap” Garand rifles used it exclusively.

From a design perspective, the entire bore pressure curve must be considered.

It does not matter whether the projectile is in the bore or not.

Only the resultant pressure curve is important.

To the original question, yes, all muzzle devices affect the pressure curve.

The difference is in how much.

IMO, most devices affect it very little, well within the normal reliability margin that most here expect out of a combat rifle.

Even some 7.62 suppressors show much less increase in gas drive than their 5.56 counterparts.

In short, it’s minutiae in most cases.

Where I think it becomes a factor is when setting up a low mass , low recoil competition gun.

In that case, all the mechanical mods should be done first and then the gas drive adjusted for the desired function.

@MistWolf

Negative. It takes time for the pressure in the rifle to drop to atmospheric after the bullet uncorks the muzzle. Good thing too, because it’s that residual pressure left in the system that allows the rifle to function

Not quite. What a suppressor does is increase the time it takes the gasses to vent the system after the bullet uncorks the suppressor. The suppressor is a pressure accumulator which slows the gas velocity and, in this case, also cools the gasses.

@jaxman7

The pressure should drop to zero upon bullet exit but something like a KAC Triple Tap I would think delay that ever so slightly. Like I said it’s probably so minimal any influence on function is irrelevant. Interesting to me nonetheless.

-Jax

[Sparky5019]

Jax,

This has tickled my brain over the years as well. It does seem that the “chamber” comps like the BC that have more rear push have back pressure vs the “open port” comps like the FSC556 have less as they release that in a larger felt side blast.

I have found no way to objectively quantify this. All of my opinions have been based on subjective measure of feel; blast, shoulder push, etc. looking at the cases from both guns with each comp, one doesn’t seem more violent by the marks on them and the ejection pattern was no different.

While I don’t own my can yet, I know that will make a subjective difference, as it should.

How can we devise a simple test to somewhat objectively quantify this with chamber vs port comps? Avoiding instrumentation would be ideal but may prove impossible here.

At the end of the day, I picked the BC because of muzzle movement and 5.56 has little shoulder recoil so a little push didn’t matter.

Clint,

If the pressure curve is everything in this context, there are a huge number of variables that go into that; powder burn rate, bullet weight, etc. Are the effects of these MDs going to have that much impact on the system given all the variables involved?

Interesting…

[MistWolf]

@Sparky5019

…If the pressure curve is everything in this context, there are a huge number of variables that go into that; powder burn rate, bullet weight, etc…

Just keep in mind that in spite of the variables, self loading rifles work just fine

[Sparky5019]

@MistWolf

Just keep in mind that in spite of the variables, self loading rifles work just fine

Oh ABSOLUTELY…I fully realize we are taking it to the nth degree here! Jax just stimulated a tickled that had been rattling around in my head for some time (it usually takes a while for things to make it around up there, lol!).

[jerrysimons]

Stumbled across this thread in which ESK designed a 9" barreled Vltor SBR with a gas system that relied on a krink type expansion chamber muzzle device/gas system to smoothly cycle the action.

https://www.m4carbine.net/showthread.php?130030-Vltor-SBR-with-FOG-what

[TehLlama]

@Sparky5019

Oh ABSOLUTELY…I fully realize we are taking it to the nth degree here! Jax just stimulated a tickled that had been rattling around in my head for some time (it usually takes a while for things to make it around up there, lol!).

Yeah, it’s minor. I’m just wondering what the barrel length equivalent in extra gas impulse is for various muzzle devices - probably something to the tune of a 10.3" with triple tap is probably equivalent to a 10.5" with something hilariously open like the YH2 phantom.

[jerrysimons]

@TehLlama

Yeah, it’s minor. I’m just wondering what the barrel length equivalent in extra gas impulse is for various muzzle devices - probably something to the tune of a 10.3" with triple tap is probably equivalent to a 10.5" with something hilariously open like the YH2 phantom.

ESK said 1" of dwell time for the BCM Gunfighter comp. He also said up to 4" added dwell time for some of the comps mentioned earlier. That’s like running midlength gas on a 20" barrel!

[BufordTJustice]

@jerrysimons

ESK said 1" of dwell time for the BCM Gunfighter comp. He also said up to 4" added dwell time for some of the comps mentioned earlier. That’s like running midlength gas on a 20" barrel!

Exactly. It can apparently make a huge difference. I’d be interested to see a cyclic rate test done for each device.

Sent from my SPH-D710 using Tapatalk

[DirtDiver06]

@MistWolf

Negative. It takes time for the pressure in the rifle to drop to atmospheric after the bullet uncorks the muzzle. Good thing too, because it’s that residual pressure left in the system that allows the rifle to function

Not quite. What a suppressor does is increase the time it takes the gasses to vent the system after the bullet uncorks the suppressor. The suppressor is a pressure accumulator which slows the gas velocity and, in this case, also cools the gasses. Slowing and cooling the velocity of the gasses before they exit increases the time it takes for the system to blow down to atmospheric and is what reduces the sound energy

Yes. Colt first used a moderator to increase blow down time so the “CAR-15” would function reliably

Is this how Salient Arms makes a “reliable” 14.5" barrel with rifle gas; by using a BC to act as an expansion chamber?