Hi .30 cal is 7.62mm...a beast of a slug!
Not sure but I think Steve Harper made some guns in .30cal?, and some of them far eastern Career/Shinsung guns may be available in that cal too?...also think there are some custom American pcps in .30cal. Giffards were available a variety big calibers, some Gems were available in 8mm. Older aircanes and butt/ball reservoir guns come in some big bores too but I don't know of any older airguns in that specific cal?