18.5" barrels are still capable of slinging heavier bullets, i.e. the aforementioned prohunters will still run 2700ish, and that is enough velocity/momentum/expansion/penetration for big deer. If you feel more confident in a larger chambering, there's nothing wrong with it, I'll simply add that the .243 can successfully take critters heavier than the muleys referenced. If the 85 gr is a mono or a frame, and it shoots great, I'd not worry much.
That sort of makes the point I was making. Is the 243 adequate for deer? The answer is it depends. Which 243 shooting which ammo on which deer at what range? Can you make it work on big deer? Sure within it's limitations. I know a guy who took at elk with one. Short range at the right angle. Do you want to take a 243 carbine out and try a shot on a 350 lb muley quartering away at 300 plus yards at dusk if you don't have to? Don't get me wrong here. I am NOT disparaging the 243. I love my carbine, but I am passing up that shot. That kind of stuff is NOT what I have a 243 for. Is it great for crossing up on coyotes and smaller deer/pronghorn/javelina at moderate ranges if you can make a reasonable shot? Sure and it's fun to shoot too.