The pressures given for loads are their peak pressures. That happens when the bullet is only one or two inches down the barrel. How high it will be when the bullet base passes over the gas port depends on the powder and bullet combination used and on how far down the barrel the port is. For charges that produce the same peak pressure, a slower powder will create higher port pressure while a fast powder will create a lower one. I think your best bet is to start with the undersize gas port and open it by incrementing the drill sizes using the load you want to shoot routinely in the gun. That lets you start with choosing the bullet and rate of twist and powder rather than adapting those choices to the gas port. It also lets you minimize the gas port so the gun has the least violent operation and drops the cases nearby rather than trying to launch them into orbit.
Something I've done with the Garand is disable the gas system and develop a load in single-shot mode, running the op-rod manually. That let's me find what the chamber and barrel like before involving the gas system disturbances. You might want to try the same thing, setting your barrel up with no gas port at all at first. Develop some candidate loads, then start drilling?
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Last edited by Unclenick; November 29, 2008 at 10:57 AM.
Reason: typo correction and clarification