Horsefeathers.... there is nothing
wrong with upgrading your rifle while using factory ammo. There are phenominal factory rounds out there with nearly ANY bullet available but there are a few caveats..
1) you need to understand what you need the bullet to do & select an appropriate bullet for the job. Then ensure your reamer is properly set up for that bullet. You are going to hinder yourself if your have a factory spec reamer & decide to run heavy for caliber bullets etc.
2) you will pay dearly for premium (no Walmart is not premium). HSM comes to mind, as well as a few other high end ammo gurus... but it will work for sure.
I'd say the 300 in your case, (although I'm kinda biased against the 7mms). RL-17 is showing fantastic velocity BUT, it is probably not going to be very temp stable (as most Alliant powder). Only time will tell.
The 300 will do just fine with a 26"-28" bbl, the ABS are pretty cool. There were some serious issue with the earlier Christensen Arms bbls, but I don't know if they are the same or not... A brake will help you spot your hits (or worse
) but will be a pain in the a%$ if you're on a coyote stand unless you use electronic protection.
Just my .002, hope it helps.