You can gain some magazine length by putting in a wyatt's mag box. You could also just seat the 300 gr hybrid to fit and feed from existing mag box. I loaded some 230 gr hybrids in my friend's 300 WSM
that had them so deep it looks like they are sitting on the primer! They shot just fine. You won't know till you try it. Remember hybrids are not seating depth sensitive like the VLDs. You don't have seat them into or close to the rifling to work. That 300 WSM's seating depth is approx .150" from rifling.