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Ok, So I have been shooting my Sig MPX kitted out for PCC. I have a ‘newer’ generation factory carbine barrel (13” +ringed birdcage). I have no problem making minor power factor with my normal 128-130PF loads for Carry Optics and Production. But, I’ve talked to some folks who are not able to use the same pistol round in their blowback AR style PCC. So, my theory is that with the blowback, enough gas pressure is lost between ignition/extraction and the bullet leaving the barrel that it is slowing down the velocity with the increased friction of the barrel. But in a gas system with a rotating/locking bolt, the timing works so that the bullet is out of the barrel before the casing is extracted due to the time it takes to rotate the bolt and extract the case. I’m sure there is some negligent bleed of gas through the gas system itself, but since that is soooo short in the MPX, I don’t even consider it. I say this because ‘normally’ most loads actually increase due to more of the powder being used in the longer barrel. In fact, in my MPX, I see a 1 or 2 point increase in PF whereas folks with blowback operated PCCs sometimes see decreases in PF. Thoughts? Steve