Gas system question

[slowjonn]

There has been a few "spirited" conversations lately regarding carbine length systems vs. mid length systems and D.I. vs. GPU's. This got me to wondering. How does a really short D.I. system work in a pistol type AR? Wouldn't this beat up the internal parts more than a carbine system? How are people getting around these shortcomings? Thanks

[ohsmily]

I am sure Randall will chime in, but my guess is that the gas port is much smaller to reduce the amount of gasses going into the tube and cycling the bolt.

[slowjonn]

Ok I can see where that makes sense. Why not do that to carbines then?

[trinydex]

it's more than a matter of where the gas system is tapped and how big the port is, it's also a function of how much barrel is left after the gas port.

the carbine length gas systems would be fine if the carbine barrels were shorter. i believe ar15barrels once said that a 12" is "perfect" (this may be an imperfect memory)

this extra barrel length makes it not only harsh because the gas system is at a point where pressure is higher but it makes the pressure last for longer.

[Dr Rockso]

Ok I can see where that makes sense. Why not do that to carbines then?

My guess (and it's just a guess) is that placing the gas block further down the barrel, you allow the powder to burn more completely and the system stays cleaner.

[trinydex]

they do modify the size of the port for carbine gas systems, the problem is the dwell time (at least that's what uclaplinker always says).

my understanding so far is that the gas port must be sized in such a way that at choke flow you get enough gas at enough force/pressure to cycle the system. this diameter is different for different pressures since flow work is dependent on pressure.

you need a certain minimum force of gas to do the flow work of cycling the gun internals. my understanding is that it's like saying a human can blow on a straw against the bolt carrier all day but it's not going to cycle the gun (plenty of gas by volume but not enough force). so you need this minimum amount of force.

i think the problem with the carbine gas length is that the port size is such that in order to achieve the minimum force you need to have it at a certain size, but the length of the barrel than prolongs this force.

edit to add question:
i'd like to add this question, is the pressure of the burn INCREASING at the point where the carbine length gas system is tapped?

[windsheer]

Does all this apply to semiauto or full auto or both ?

[ar15barrels]

https://www.ar15barrels.com/prod/operation.shtml

[ar15barrels]

i'd like to add this question, is the pressure of the burn INCREASING at the point where the carbine length gas system is tapped?

oh no.
bore expansion ratio is far exceeding the pressure rise and most powder is consumed well before a carbine gas port.

[ar15barrels]

Does all this apply to semiauto or full auto or both ?

both.

[ar15barrels]

Wouldn't this beat up the internal parts more than a carbine system? How are people getting around these shortcomings? Thanks

oh yes.
The key is to have the right combination of port size and dwell time.
Using a 6.5" to 7.5" barrel with a pistol gas port location is MUCH better than a 10.5" barrel with a pistol gas port location.

[ar15barrels]

my guess is that the gas port is much smaller to reduce the amount of gasses going into the tube and cycling the bolt.

That's pretty much it.
Every combination of gas system length and barrel length has a different port size requirement.
You can't just look at a 20" rifle and measure it's 0.093" port and expect that to work when you have a 16" barrel with a carbine gas system.
It's SO different.

My operation page has some predictions in bullet travel time and gas port average exposure pressures that show the differences.
These numbers won;t ever correlate directly to a port diameter, but they tell most of the story.

I have done so much experimenting with port sizes, barrel lengths and gas system lengths that I can pretty much SWAG a port diameter now and it runs correctly.

[ar15barrels]

Ok I can see where that makes sense. Why not do that to carbines then?

Ammo is not standardized.
Even worse is that customers are not standardized.

If I de-gassed a barrel and a guy runs sub-par wolf ammo, it will short cycle and I get the blame.
The customer will generally disavow any wrongdoing.
Also, customers will put a heavier buffer in a gun than it really needs because someone told them they needed it.
The heavy buffer re-times everything and all of a sudden, the guy that made the barrel is at fault again, even though it works when used with a standard buffer.
Therefore, most carbine ports will continue to be drilled oversized and the companies let the customers fix the problems...

I have made carbine barrels with port sizes 3-15 sizes smaller than the standard (colt) that run fine with proper ammo and standard buffers.

[trinydex]

oh no.
bore expansion ratio is far exceeding the pressure rise and most powder is consumed well before a carbine gas port.

ok cool, good to know the scale on which these things happen

[ar15barrels]

ok cool, good to know the scale on which these things happen

My write-up has a graphic showing bullet travel in relation to pressure.
It's easy to see where the peak is.