Ive had two 300 PRCs built, both on Kimber Montana 8400 (300 wm donors) both had the mag box lengthened to 3.750" , the first one we used a 26" Hells Canyon Armoury Carbon Fiber wrapped barrel, 1-9.5 twist, High comb and palm swell added to the Montana stock, CF bolt knob added, this gun turned out awesome and shot everything I stuffed in it well with 76.0grs of R26, factory ammo shot well and velocity was as advertised.
The second one is basically the same as the first one but we used a 26" #4 Fluted Benchmark 1-9.5 twist barrel, this one seems to be a bit faster velocity wise than the CF barrel, Im shooting factory Hornady 212gr ELDX ammo 100 fps faster than advertised (2860fps), Im getting 2960-2970 fps and the 225gr ELDM (adv 2810) and Im getting 2910fps avg
Ive shot the 225gr ELDM factory ammo out at 500, 600, 720, and 820, my first group at 820 yards was 3", my gunsmith was watching through the spotting scope and he says "wow I need one"