AH senior member
With the R8’s it’s mostly stock type one prefers, magnum vs non-magnum bolt head though those can be swapped out. You can only get above a 375 H&H with the traditional type stocks, not the thumb hole style. The rifle is designed to be modular but there are some limitations. There are quite a few barrel options available as well.
I have a Professional Success and Ultimate, both of which are thumb hole. You can get a .375 RUM in these, which is a vicious round. My .375 has worked well on a variety of DG with the thumb hole stocks. Unless you're hunting elephant, a .375 in H&H, Ruger, Blaser or RUM is all you need.
You can get a custom barrel in any caliber and other customization imaginable. A 28" barrel, needed for ultra high velocity rounds, works great in these stocks.
For the bigger calibers, .416 and above, you need the traditional safari stock.