Ar overgassing

[G.B.]

I don't quite understand this. If an ar has a .750 gas port and a carbine set up how is the pressure different from one manufacturer to another.

[truename]

.750 is the diameter of the barrel where the gas port is, not the diameter of the gas port. The diameter of the gas port itself varies by manufacturer.

[G.B.]

So there is no spec for a gas port for them to follow.

[EW1066]

I may be off base, but I believe that there is no difference in pressure, as the gas port does not regulate pressure. It does however control the speed at which the pressure builds. So while the pressures are the same the onset of maximum pressure is inversely proportional to the gas port size. That is my understanding, based on what I have read here on this sight. If I'm wrong someone will be along shortly to correct me.

EDUB

[varmint49]

stander gas port hole size is .063

[C4IGrant]

So there is no spec for a gas port for them to follow.

Yes, there is a spec for 14.5" carbines and 20" Rifles. Every other length and gas system is generally a "guess" by the manufacturer.

Some of the better companies (like DD, BCM, etc) do cyclic rate testing, etc to fine tune their GP sizes (like on Middy's that have no TDP).

Mil-Std GP size on a 14.5 carbine is .063. A 16" carbine should also be around .061-.063 as well.

Your BM, RRA, Oly, DPMS, S&W, etc with a 14.5-16" barrel and a carbine gas system are all over gassed per the TDP.


C4

[Todd00000]

Yes, there is a spec for 14.5" carbines and 20" Rifles. Every other length and gas system is generally a "guess" by the manufacturer.

Some of the better companies (like DD, BCM, etc) do cyclic rate testing, etc to fine tune their GP sizes (like on Middy's that have no TDP).

Mil-Std GP size on a 14.5 carbine is .063. A 16" carbine should also be around .061-.063 as well.

Your BM, RRA, Oly, DPMS, S&W, etc with a 14.5-16" barrel and a carbine gas system are all over gassed per the TDP.


C4
What is TDP?

[Failure2Stop]

What is TDP?

Technical Data Package.
List of government requirements and testing for weapons.
Whole lot of info on it if you search or google it.

[Todd00000]

Technical Data Package.
List of government requirements and testing for weapons.
Whole lot of info on it if you search or google it.

Thank you. So my 16 inch DPMS is over gassed, but it fires fine and I am accurate with it, is there anything I need to do?

[Failure2Stop]

So my 16 inch DPMS is over gassed, but it fires fine and I am accurate with it, is there anything I need to do?

Overgassing can lead to a few different problems.
Ripped case head
failure to extract
failure to eject
failure to lock the bolt to the rear on empty magazine
bolt override
snappy recoil
early spring wear
bolt bounce

A few of these issues are band-aided by:
Extractor O-ring
F/A BCG
H or H2 buffer

Seems that most manufacturers that over-gas are due to their primary market segment's use of cheap ammo, which can cause issues with properly ported guns.

If it isn't giving you any issues, crack on, but if you do see issues coming up, you know how to fix it.

The sticky:
Oh no, I bought a (whatever) gun
has a lot of good info on how to fix these problems if they do arise.
ETA- here's the link https://www.m4carbine.net/showthread.php?t=7376

FWIW- porting has almost nothing to do with precision

[C4IGrant]

Thank you. So my 16 inch DPMS is over gassed, but it fires fine and I am accurate with it, is there anything I need to do?

Depends on the definition of "fires fine." For me, no it does not. It has way more felt recoil than it should.

When a gun is over gassed, you get more carbon, more felt recoil and more wear on parts.

I would put a quality buffer spring and an H2-H3 buffer in it to slow things down.


C4

[Todd00000]

Overgassing can lead to a few different problems.
Ripped case head
failure to extract
failure to eject
failure to lock the bolt to the rear on empty magazine
bolt override
snappy recoil
early spring wear
bolt bounce

A few of these issues are band-aided by:
Extractor O-ring
F/A BCG
H or H2 buffer

Seems that most manufacturers that over-gas are due to their primary market segment's use of cheap ammo, which can cause issues with properly ported guns.

If it isn't giving you any issues, crack on, but if you do see issues coming up, you know how to fix it.

The sticky:
Oh no, I bought a (whatever) gun
has a lot of good info on how to fix these problems if they do arise.
ETA- here's the link https://www.m4carbine.net/showthread.php?t=7376

FWIW- porting has almost nothing to do with precision

Thank you and I'll read the thread; but that's what I'll do, shoot the shit out of it until something breaks.

[Todd00000]

Depends on the definition of "fires fine." For me, no it does not. It has way more felt recoil than it should.

When a gun is over gassed, you get more carbon, more felt recoil and more wear on parts.

I would put a quality buffer spring and an H2-H3 buffer in it to slow things down.


C4

I'm prepared to do that. I have fired it back to back with my 6920 and not noticed a big difference. The Army has made me a good shot and adaptable. I think that makes me overlook differences and concentrate on the fundamentals to stay on target, but now I know what to look for, thanks again.

[Texas42]

Learn something new, every day.

[BufordTJustice]

Depends on the definition of "fires fine." For me, no it does not. It has way more felt recoil than it should.

When a gun is over gassed, you get more carbon, more felt recoil and more wear on parts.

I would put a quality buffer spring and an H2-H3 buffer in it to slow things down.


C4

+1

Simply functioning is failing to draw a finer distinction between running well and just running.

My wife's econo-car starts every time she turns the key and 'runs well'. I still would not compare it to a BMW or Lexus....both of which also start and 'run well'.

An H2 buffer here:
http://www.gandrtactical.com/cgi-bin/commerce.cgi?preadd=action&key=GRT-HB&reference=/cgi-bin/commerce.cgi%3Fsearch%3Daction%26keywords%3Dbuffer%26searchstart%3D9%26template%3DPDGCommTemplates/FullNav/SearchResult.html

And a CS Springco BLUE spring here:
http://www.gandrtactical.com/cgi-bin/commerce.cgi?preadd=action&key=25006&reference=/cgi-bin/commerce.cgi%3Fsearch%3Daction%26keywords%3Dbuffer%26searchstart%3D0%26template%3DPDGCommTemplates/FullNav/SearchResult.html

would serve the OP well if he has an overgassed carbine.

[jmart]

The issue of how it runs and if it runs properly is an academic debate. If you end up adjusting for the "overgassed port" with a heavier carrier, buffer and/or spring, and achieve a resultant 750-800 rpm cyclic rate, the fact that it's overgassed is irrelevant. Overgassing only becomes relevant when cyclic rates are high, and there are ways to mitigate that.

I don't buy into the notion that a properly gassed 6920 running at 750 rpm operates better than an overgassed BM that's equipped with the above and also runs at 750 rpm. 750 is 750.

[Clint]

The issue of how it runs and if it runs properly is an academic debate. If you end up adjusting for the "overgassed port" with a heavier carrier, buffer and/or spring, and achieve a resultant 750-800 rpm cyclic rate, the fact that it's overgassed is irrelevant. Overgassing only becomes relevant when cyclic rates are high, and there are ways to mitigate that.

I don't buy into the notion that a properly gassed 6920 running at 750 rpm operates better than an overgassed BM that's equipped with the above and also runs at 750 rpm. 750 is 750.


Yes and no.

High cyclic rate is the primary bad result of over gassing that leads to early extraction under high chamber forces.

Also, to be clear the high cycle rate is due to higher piston pressure and acceleration of the BCG.

Sure, you can lower the cyclic rate by adding extra mass.

This does cure the early extraction, but the higher piston pressure remains.

So you've got more pressure driving a higher mass at the same speed as a properly gassed setup.

[straightsilver3]

http://www.defensereview.com/the-big-m4-myth-fouling-caused-by-the-direct-impingement-gas-system-makes-the-m4-unreliable/ The article has some info on gas port size and over gassed and possible fixes

[Todd00000]

Ordered an H2 buffer from G&R Tactical. I'm going to try it out in the DPMS and 6920.

[constructor]

Thank you. So my 16 inch DPMS is over gassed, but it fires fine and I am accurate with it, is there anything I need to do?

You can if you have tools, remove the gas block.
Tap the hole between the barrel and gas tube, then install a set screw in the space between the barrel and gas tube. drill a .0625 or 1/16" hole through the set screw. A reducing sleeve like that will give you more consistant results than a adjustable gas block with a set screw coming in from the side.
Heavier springs and buffers also help but it is still over gassed and starting to move the carrier too early which can result in bad accuracy.
You can shoot your rifle in the dark, if flames come out of the ejection port then the carrier is moving rearward while the bullet is still in the bore and some pressure is exiting through the chamber. Some rifles are severely over gassed.

[Todd00000]

You can if you have tools, remove the gas block.
Tap the hole between the barrel and gas tube, then install a set screw in the space between the barrel and gas tube. drill a .0625 or 1/16" hole through the set screw. A reducing sleeve like that will give you more consistant results than a adjustable gas block with a set screw coming in from the side.
Heavier springs and buffers also help but it is still over gassed and starting to move the carrier too early which can result in bad accuracy.
You can shoot you rifle in the dark, if flames come out of the ejection port then the carrier is moving rearward while the bullet is still in the bore and some pressure is exiting through the chamber. Some rifles are severely over gassed.
Thanks I'll check that out also, but I'd buy a new barrel before going to that trouble. I'm not too worried about it, I have a 6920 and a few lowers for future quality builds. I just like knowing how and why things work.