300 WSM simply because it has 2-3x the barrel life of other mags and as long as you do not go above a 210, it will hang with any of them speed wise and definetely accuracy wise. With a 30" barrel they will shoot a 210 2950-3050 fps and at 1000, sub .5 MOA for 5 shot groups consistently, not 3 shot once in a lifetime.
Short action, wyatts box, sell the existing bolt and buy a Kiff bolt with sako or M16 extractor.
You also might want to talk to Dave Kiff at Pacific Tool and Guage about the 280 HK. It is a modified 280 AI, that the Hornday techs and Kiff came up with and put together, ergo the "HK". The Hornady guys are shooting it and doing extremely well in comps with the Hornady 168. Pretty sure also that Hornady can get you dies asap.