ActuallyI was going to suggesta .257 calbut you seemed pretty set onscratching an itch, so didn't
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Can't really compare to the 300 RUM as it is better suited for larger game. However for sheep, deer, antelope and varmits I really don't think you can go wrong with a 1/4 bore. Recoil is nothing, easy to load for and very effecient cartridges (anything the 270 can do IMHO). While I have been witness to the standard 2506 taking bull moose, bull elk and black bears I think their are better choices, choices for which you are well stocked already. I regret that the day I sold my rh 2506 I didn't replace it immediately with a LH version. I am looking for a donor Lh action to build a 2506AI, as i like the idea of sending a 110-115gr quality bullet down stream in the 3400fps area. If interested in such I suggest you PM Roskoe, he has a 2506AI and am sure he'd be willing to share his finding on its potential and prowess.
G/L