A while back I kinda wondered roughly the same thing. because of the lighter construction of a 30-30 projectile designed to perform at 30-30 velocity. I have a ton of 30-30 150 grain prohunter I got from a guy for few bucks a box got like 400 projectiles total. Anyway back to what i was typing Ive been pushing these at full throttle from my 308 with ok accuracy good enough for hunting not gonna win any bench matches anytime soon with'em. On coyotes the bullets blow apart like a V-max and do heck of a job knocking their *beep* in the dirt.

but yeah its perfectly fine to load 30-30 projectiles in a 308. Infact the 307 which is a rimmed 308 actually uses 30-30 projectiles.