The XP-100 is a cool gun. I passed up several in bigger calibers, and with custom stocks until I found one of the original .221s with the original stock.
A sweet trigger, and as accurate as a bolt action rifle.
For many years I couldn't figure out why Rem made the .221, why not just chamber it in .222 or .223?
Then, it dawned on my, after comparing load data and velocities.
It was the 10" barrel of the XP-100 that was the real reason. The .221 Fireball case holds all the powder that will be useful in a 10" barrel. Both the .222 and .223 burn more powder but don't give any real increase in velocity, from a 10" barrel.