Depend a bit if you handload or are going to primarily buy factory ammo. 243 isn't a bad round. You'll probably burn the barrel out in 1600 rounds or less.
If shooting factory ammo: 6.5 Creed, 6 Creed, .308 (TAC class) are your best options.
If you hand load, the options are somewhat endless. The 6.5 Creed and 6.5x47L are the two main 6.5mm options you see at a match. More and more people are shooting 6mm, or even 22 caliber. 6 GT is gaining popularity, 6BR, 6BRA, 6mm Creed is still used a bit but it's so finicky and burns barrels very quickly so I've seen alot of competitors move away from the 6 creed.
PRS is a recoil management game. The worst thing you can do is pick a large round with a heavy bullet that generates significant recoil.
You want your gun to have some significant weight (most guns are in the 16 to 22lbs range), but also balance well. The balance point should be a few inches in front of the mag well.
I' started with the 260 Ackley Improved round, and shot 2 seasons with that rifle. Awesome ballistics, but the sharp shoulder created feeding issues, and the extra powder over a standard .260 created more recoil.
The prior 3 seasons I've moved to the 6.5x47L. It's not the sexiest ballistics, but it's insanely consistent and easy to load for which means less time doing load development and tuning the load or rifle. I developed one load for the 6.5x47L using Berger 130's and after 2500 rounds the accuracy, velocity and sd/es never changed. That consistency is valuable to me as I just don't have much time to chase accuracy loads or sd/es.
Start with what you have and shoot some local matches. Come out and learn and have fun. It's an addicting sport!
