You can get one in 6mm too .
Yes, yes...the "spherical composite-material projectile" ammo...
take a look at this might interest you http://www.specterarms.ca/ non restricted
^ 6mm Plastic BB
.177 cal pellet (airgun)
AND 4.5mm Metal copper BB (theyre stronger than one thinks, and becuase of steel projectile, they go threw car windsheilds.