[eth77]

That's what I was looking for, but actually wanting to avoid calculating.

I figure that if I have to do it, I'll look up the pressures for some reloads on AmmoGuide Interactive. That will give me some gas pressures that I can work with. What I won't know, is where the bullet was when that pressure was reached. Or is that the pressure as the bullet exits the barrel?

Next I need to calculate the volume of gas I'm going to need to tap. start with the min volume for the tube - put a piston into a gas adapter after filling the adapter with water, then measure the remaining water after the piston hits bottom. That gives me my starting point, then measure the piston chamber volume just as the piston reaches the bleed ports - that's my max volume.

I'm going to guess that the port sized was picked so that the weakest load would cycle the action, thus a good start would be a light bullet at relatively low velocity.

But I really want to just use an equivalent of the current gas adapter on a port which would be the same size and 1" farther forward. I need a new adapter, as the barrel will be 0.800" and the current barrel is 0.765" in diameter. I'll also need a new piston, longer by 1". While I'm at it, I'll make it a smidgen sturdier - make the shaft between the two ends slightly thicker. This will make it heavier, so it may take more gas, but that can be finessed with a slightly larger port.