Factory airforce guns come with a threaded tophat stem to adjust how far it sits in the breach. Airforce told me it should sit as far forward as possible while still allowing the bolt to fully close. But there are many aftermarket valves/regulators/stems with non-adjustable tophat depths. Which begs the question... how much does tophat depth actually matter?
I adjust mine so that when closed, the breech is snug against the hat, which sounds exaclty how Airforce advised you. That way there is minimal air bypass. For aftermarket hats, as long as they are consistent and don't leak too much air, there might not be a problem. Apparently the top hat clearance was one way of adjusting power before the power wheel was introduced:
I also add an o-ring under the hat if the orifice size is a little large for the power level I desire, or remove it if I want more power, say, for heavier projectiles in that caliber.
