The heaviest 6mm BBs you can get are .88g steel ones, made by PPProducts (makers of the strafer and minigun). They require no modification at all to fire, but if your gun isn't powerful enough, they won't go very far. Also, steel is a lot more likely to mark/damage the inside of your inner barrel than plastic BBs.
I suppose technically, a .22 cal pellet would fit in the barrel (it would be somewhat undersized, if I'm not mistaken), if you were either to muzzle load it or load it through the open bolt of a Tanaka sniper rifle. A Tanaka is pretty much guaranteed to fire it hard enough to matter as well.
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