We are selling a lot of (50) 50 Caliber .510” Maxi Ball pellets which have good rifling contact with less friction.
Almost every modern problem that we find with pellet designs and power out of PCP air rifles has been addressed in the last couple hundred years when much older musket rifles and similar and much less power and wanted to optimize it. The answers were simple enough, lube the bullets and come up with creative designs to maximize power like mini-ball, maxi-ball, and hollow skirt bullet designs.
Keep the gas charge of the bullet being fired behind the bullet with minimal gas bypass and you will have more power. So, make a bullet too tight and you increase barrel friction and reduce power. Make the bullet too loose and you have gas bypass and less contact with the lands and grooves (Slower and less accurate). Make the bullet the right size and minimize rifling contact by spreading out a few bands and it will be stable (Accurate) and have less overall friction with the barrel.