Whats the Science behind shooting heavier projectiles? (like .247, .30, .357)

[39M]

Yeah I saw that Paco Kelly thing a while back. But I just shoot'em stock.

About that Eley Ammo, yeah the rim is part of it.
They also have a special way they make the primer flow out and meter it very precisely. There's even have a special name for it. They say the primer is a critical factor because it provides more propellant than the powder does.

And then they do all kinds of other crazy stuff like checking the humidity and temperature in the place where the powder comes from on the day they made a certain batch.