SpudFiles

i just need confirming with some help on projectiles

what are the ideal size of projectiles for an air gun using an hammer valve?

Would it be able to launch marbles or anything around around that size
(15-19mm) with lots of power?

It depends on many parameters, namely strength/weight of the hammer, valve porting, chamber pressure and barrel length.

The Dragon Slayer for example can throw a 0.5" 13 gram slug at just under 600 feet per second, giving it more muzzle energy than a 22LR firearm - but it does this with 3000 psi in the chamber.

For lower pressures typically used in airguns I wouldn't really use a hammer valve directly, rather as a pilot for a QEV style valve, as BtB did here.