bighammer,

Based on all the responses you've received, It looks as though the ideal step up from a .375 is to purchase a .416 Rigby AND a .458 Lott.

This option is entirely reasonable unless it inflicts excess harm to the bank account.

Regarding rebarrelling to .450 Rigby, if it is a Ruger 77 RSM, rebarrelling will mean tossing out a barrel that is VERY expensive to replace..the quarter rib is machined as a part of the barrel. Reboring or simple rechambering in the same bore size is the only real option there because a custom gunsmith might charge USD $2,000 or $3,000 just to make a barrel with the integral rib.

Richard.