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).
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