I have no experience with teh 243, but I can tell you that 7.62x39 is a fun little cartridge for plinking and perfectly suited for deer out to 150 yards.

I have killed pigs with it, but would not attempt to drop a big bore beyond 150 yards. The cartridge just doesn't have enough power to penetrate big shoulders beyond that distance. I probably would not even shoot at deer any farther than that.

It works very well for deer within 100 yards.