Hey guys, sorry if this is the wrong place to ask this. I just have a quick question that I can't really find an answer to online. I want to know what the minimum gas port pressure is to correctly cycle an AR with stock buffer, bcg, and spring. I can run factory loads in quickload, but that doesn't really tell me the minimum, and I don't want to model it in SolidWorks and wait 400 hours for a simulation to solve it. It doesn't have to be super exact, I just want to get an idea of what range it is in. Thanks.

Dano posted this in the other gas issues thread.

11.5" barrel, .625: dia., 3.850" from muzzle;
Port sizes from .081" to .089"
11.5" barrel, .750" dia., 3.850" from muzzle
Port sizes from .086" to .094"

14" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .063" to .078"
14" barrel, .750 dia., 8.375" from muzzle;
Port sizes from .070" to .086"

16" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .063" to 078"
16" barrel, .750" dia., 8.375" from muzzle;
Port sizes from .070" to .086"

20" barrel, .625" dia., 8.375" from muzzle;
Port sizes from .086" to .093"
20" barrel, .750" dia., 6.875" from muzzle;
Port sizes from .093" to .096"
24" barrel, .825" dia.,
Port size .089"

"Stock " is relative.

Ever brand calls stock something different including milspec guns.

A colt 6920 would probably be the best example to use.

Thanks for the port sizes, but doesn't really help much in my situation.

For stock I was just referring to nothing extreme. Say 5.56 in a mid length gas system. Not a 300blk suppressed pistol or something. I should have been clearer.

For the pressures at the different spots in the barrel and gas system... Chris Bartocci might be the source to try.

IIRC, the pressure in the chamber is around 55k psi for 5.56. Don't be confused by the 62k psi PROOF level. Pressure dissipates and again, IIRC, needs to be around 20-22k psi when it gets around and back to the gas key to cycle everything.

So no matter the port size and how far down the barrel it is, just needs to be a certain amount when it gets back to the carrier.

What can change that is buffer weights, carrier weights, modified cam paths, extra bleed off in the system (like carrier, as the gas block usually has a carbon seal).

For the numbers, again, I'd say try to get in contact with Chris Bartocci or research his blog posts. Might be something. I'd also say SUBSCRIBE and support.

That or dig around the net. Probably is you really need a lab to get these numbers. Chris B spends time in Black Hills' lab.

Sorry for still not answering your question up front.
The question has a lot of variables. Gas port size, gas system length, barrel length, brass or steel casings, how does the carrier / bolt react to pressures, how freely does the carrier cycle, buffer and spring weight, how stable is the rifle held etc.?
The answer would be different for each rifle and be different by changing one small thing. I would guess around 25,000 PSI port pressure but that is not scientific.

I was able to get my 14.5" (carbine gas system) barrel to cycle reliably to about 42000 PSI (based off Quickloads data) using 55 grain FMJ.  Can not get QL to load on my Windows 10 computer so the actually load is on another machine (monitor blew).  I can not recall which end of the burn rate spectrum I was using, but it was a powder at either end........For some reason I think it was a slower burning powder which filled the case more resulting in a more even burn and slower pressure spike.

That said, I was using a standard carbine system buffer system and a old school AR15 carrier without firing pin shroud and semi auto profile.  Definitely the lightest carrier I had available.

I believe the original rifle gas port pressure was supposed to be ideal @ 14,500 c.u.p +/- 2000.

The reasons to reduce port diameter with shorter gas systems is two fold.

1. Pressure is much higher the closer you get to the chamber.

2. Timing is changed because the bolt is being unlocked more quickly after the round has been fired. This causes the brass to be still under significantly more chamber pressure when the bolt is trying to open.

Heavier buffers slow the system down enough so the brass has a few more miliseconds to contract after being fired.

If you are ripping up case rims on extraction you need a heavier buffer.

I do a lot of wildcats with the AR and the numbers I use on picking a powder for proper function, which has never let me down yet, is 12K to 14K psi at the gas port, regardless of GP location, and 2-4K psi at the muzzle.  YMMV.

Nvmd.

IIRC the 20"guns needed ~13,500psi with the gas flow volume from the original gas port diameter.  I can't remember where exactly I read it... Think it was 10+ years in a Small Arms Review magazine article.

advntrjnky

"We often hear about mid-length being smoother cycling or pistol being harsher cycling than the typical carbine length gas systems.
Below is a plot of a 223 load.
I have noted the locations of the various gas ports in blue.
You can plainly see what pressures are introduced into the gas systems when the bullet JUST passes the gas port.
This is the reason for the way the various gas system lengths function differently.
The longer the gas system from the chamber, the smoother the gun runs, allowing for the fastest double-taps without the gun jumping off target.
Projectile travel at the bottom assumes that the bullet starts out about 1.5" from the breech, so add 1.5" if you want to compare velocities at different lengths."

Mil standard for M855 out of an M4 is not less then 15,300psi at the port.

M16A2-A4 is not less then 13,000psi at the port

M193 was expected to be 15k +/- 2k in an M16A1.

And people need to stop calling M855A1 a "proof" round. It's still 8kpsi off of a proof round (70kpsi for M197)