Well, as noted, it is capable of 300 and 400yd. on deer, but the terminal ballistics are getting pretty low that far out. As anyone can tell from other comments I've made elsewhere, I figure the 243Win. to be best used as a varmint rifle/cartridge. Yes, it
can do for deer, but IMHO it is MINIMAL. I'd limit the range to 200yds., 250max.
Use premium ammo/bullets (ex.: nosler partitions), and broadside heart/lung shots only.

If you want long range performance for deer, get a 270Win., 270WSM, or 7mmRem.Mag.