Gas Tube Pressure

[prepare]

Anyone know approximately how much pressure goes through the gas tube to unlock the bolt and push the BCG rearward?

[GH41]

A bunch.

[DG23]

A bunch.

Pretty sure it is closer to a crap ton than a bunch.

[titsonritz]

Enough to cut your finger, I'd wager.

[titsonritz]

Mil-spec. 20-inch AR15 calls for 12,500 psi...

according to...

http://www.mssblog.com/2018/08/09/re...20dandy%20with

and

https://www.ssusa.org/articles/2020/...ey-to-function

Approximate figures for M855 (genuine 5.56 NATO) chamber pressure is about 60,000 PSI; pistol-location gas port pressure, 50,000; carbine-location, 33,000; mid-length, 27,000; rifle-location, 19,000.

[prepare]

Mil-spec. 20-inch AR15 calls for 12,500 psi...

according to...

http://www.mssblog.com/2018/08/09/re...20dandy%20with

and

https://www.ssusa.org/articles/2020/...ey-to-function

Approximate figures for M855 (genuine 5.56 NATO) chamber pressure is about 60,000 PSI; pistol-location gas port pressure, 50,000; carbine-location, 33,000; mid-length, 27,000; rifle-location, 19,000.

Thanks for the info and links!

[win&legend]

Powder burn rates and projectile materials / design (malleability of the jacket / core) and projectile mass all affect port pressure. So assume some variance of a few thousand PSI depending on the load.

Gas port size, gas port location (obviously as stated above), BCG and buffer mass along with spring tension (buffering potential) all affect dwell time and consequently average port pressure.

You can get instances of very high peak port pressure but inadequate dwell time and still have cycling issues. My suggestion is to use a tuning load, a know good quantity and tune all rifles to that load.

This is an excellent article regarding port location vs. port pressure, tested in 1 inch increments on a test barrel that cut off in 1 inch sections at a time. Tuning load was Lake City M855 kept chilled in a cooler: https://ndiastorage.blob.core.usgovc...hilipDater.pdf

According to the US governments testing we would have the following port pressures with M855 (these are averages over 5 test shots):

Rifle Length (12inches) @ 9,815 PSI
Mid-Length (9inches) @ 13,567 PSI
Carbine Length (7 inches) @ 17,040 PSI
Pistol Length (4ish inches) @ over 25,000 PSI

You can see why both Rifle Length and Mid-Length gas systems have a reputation for "soft recoil impulse" because mass has little to do with recoil energy, it's gas port pressure and dwell time that affect how much energy is transferred into the BCG. The area under the pressure curve is your available recoil energy as it pertains to the BCG.

Mass affects rates of acceleration and deacceleration as well as average carrier speed. Generally, shooters perceive broader waveforms (think gradual but long rainbow arc) as "lower recoil" than short but peaky (think mountain tops) recoil wave forms. Buffers like the "hydraulic buffer" do nothing to actually change the recoil energy dumped into the BCG, instead they re-shape the waveform into a table top (think a digital square wave), making it linear.

But adjustable gas blocks are a boon for gas port tuning, if your over gassed (either the port is too large or the location is not optimal), you can reduce gas flow via a restricting orifice and consequently cause a pressure drop, so you can reduce the energy dumped into the BCG (this is where low mass carriers come into play, the lower weights give you more ideal carrier velocity on less gas port pressure).

Also note that testing found that the further away from the chamber the gas port is, the less variation in peak pressure there is (more consistent cycling pressures). You also have lower velocity losses due to gas leakage, obviously because at lower pressure there's less flow through any leak points (a properly built upper should have very low leakage in the first place, but anything we can do to increase velocity with affecting reliability or accuracy is a plus).

[prepare]

Powder burn rates and projectile materials / design (malleability of the jacket / core) and projectile mass all affect port pressure. So assume some variance of a few thousand PSI depending on the load.

Gas port size, gas port location (obviously as stated above), BCG and buffer mass along with spring tension (buffering potential) all affect dwell time and consequently average port pressure.

You can get instances of very high peak port pressure but inadequate dwell time and still have cycling issues. My suggestion is to use a tuning load, a know good quantity and tune all rifles to that load.

This is an excellent article regarding port location vs. port pressure, tested in 1 inch increments on a test barrel that cut off in 1 inch sections at a time. Tuning load was Lake City M855 kept chilled in a cooler: https://ndiastorage.blob.core.usgovc...hilipDater.pdf

According to the US governments testing we would have the following port pressures with M855 (these are averages over 5 test shots):

Rifle Length (12inches) @ 9,815 PSI
Mid-Length (9inches) @ 13,567 PSI
Carbine Length (7 inches) @ 17,040 PSI
Pistol Length (4ish inches) @ over 25,000 PSI

You can see why both Rifle Length and Mid-Length gas systems have a reputation for "soft recoil impulse" because mass has little to do with recoil energy, it's gas port pressure and dwell time that affect how much energy is transferred into the BCG. The area under the pressure curve is your available recoil energy as it pertains to the BCG.

Mass affects rates of acceleration and deacceleration as well as average carrier speed. Generally, shooters perceive broader waveforms (think gradual but long rainbow arc) as "lower recoil" than short but peaky (think mountain tops) recoil wave forms. Buffers like the "hydraulic buffer" do nothing to actually change the recoil energy dumped into the BCG, instead they re-shape the waveform into a table top (think a digital square wave), making it linear.

But adjustable gas blocks are a boon for gas port tuning, if your over gassed (either the port is too large or the location is not optimal), you can reduce gas flow via a restricting orifice and consequently cause a pressure drop, so you can reduce the energy dumped into the BCG (this is where low mass carriers come into play, the lower weights give you more ideal carrier velocity on less gas port pressure).

Also note that testing found that the further away from the chamber the gas port is, the less variation in peak pressure there is (more consistent cycling pressures). You also have lower velocity losses due to gas leakage, obviously because at lower pressure there's less flow through any leak points (a properly built upper should have very low leakage in the first place, but anything we can do to increase velocity with affecting reliability or accuracy is a plus).

Thanks for the additional info and link!

[lysander]

The pressure at the carrier key is very much lower than the port pressure and an order of magnitude lower than the chamber pressure. Even though the pressure at the port in the barrel is much higher in the carbine gas system compared that of the rifle gas system, the ultimate pressure in the carrier cavity is almost exactly the same in both cases, around 20 MPa (3,900 psi). This is due the the fact that the gas tube, even the short carbine tube, chokes the flow and thus restricts the mass allowed to get to the carrier.
Since the mass flow is roughly the same, the resultant pressure is about the same.

The red line is the pressure at the bullet base. The yellow line is the pressure at the barrel port. The blue line is the pressure inside the gas tube at the front sight, this peaks at 5,500 psi for the rifle and 3,600 psi for the carbine. The drop is due the restriction of the gas port. The lower black line in the pressure at the carrier key, both around 20 MPa. However, you should note that in the carbine system the pressure the pressure appears earlier and ramps up faster.

[prepare]

The pressure at the carrier key is very much lower than the port pressure and an order of magnitude lower than the chamber pressure. Even though the pressure at the port in the barrel is much higher in the carbine gas system compared that of the rifle gas system, the ultimate pressure in the carrier cavity is almost exactly the same in both cases, around 20 MPa (3,900 psi). This is due the the fact that the gas tube, even the short carbine tube, chokes the flow and thus restricts the mass allowed to get to the carrier.
Since the mass flow is roughly the same, the resultant pressure is about the same.

The red line is the pressure at the bullet base. The yellow line is the pressure at the barrel port. The blue line is the pressure inside the gas tube at the front sight, this peaks at 5,500 psi for the rifle and 3,600 psi for the carbine. The drop is due the restriction of the gas port. The lower black line in the pressure at the carrier key, both around 20 MPa. However, you should note that in the carbine system the pressure the pressure appears earlier and ramps up faster.

This is very interesting.

How much of this was known at the time the AR was designed?