[Radny97]

Jimbob86 check out ballistics by the inch or some other data before you rolleyes at me *wink wink*
I'm speaking of factory ammo, not reloading. The additional velocity comes from the barrel length, not reloading ammo in an unsafe way. You can also reference the buffalo bore website and look at the velocities they have listed for their ammo out of a lever gun. It rivals an anemic factory 30-30. The numbers are there if you care to look at them. You can ignore the numbers if you want, but these are published velocities.
Buffalo bore 125 gn 357 mag does 2298 fps out of a 18.5 inch marlin (see their info for item 19D/20)
Buffalo bore 158 gn 357 mag does 2153 fps out of the same gun. (Item 19C/20)
Remington core-lokt PSP 125 gn 30-30 does 2175 fps from a 20 inch rifle (see ballistics101.com)
Remington copper solid tip 150 gn 30-30 does 2220 fps (ballistics101.com)

So . . . yes a hot factory 357 mag from a lever action WILL rival an anemic 30-30 load. But, as I said, the ballistics of a 357 bullet shape means it loses velocity more rapidly and does not perform as well at longer distances.