For deer, a 243 is about ideal. If you want a multipurpose cartridge for deer and larger game one of the 6.5's is the smallest I'd feel comfortable with. I've had a 243 in the past but let it go. I could do everything I need to do with a 223 and either a 7-08 or 308. I consider the 308 and 7-08 ballistic twins. On paper the 7-08 looks slightly better, but in the real world no one will ever notice. I chose 308 before I started hand loading. If not I might have gone 7-08.
I did give the 6.5 CM a try a few years ago. I do like the round and it lives up to the hype. But if you're keeping the 243 I'd skip it for 7-08 or 308.
