MauserRifle has it pegged - go with a Mauser actioned .375H&H - absolutley the best idea - It also must feed slam-fed as well as up from the magazine. If the rifle doens't do this, the extractor can be re-ground and polished to allow slam-feeding. (throwing a round onto the follower and chambering it)
: The ZKK602 BRNO is the ultimate big Mauser action, today, bar none. IMHO, of course. Mine held 235 thorugh 350gr. in a 2 1/2" circle at 100 meters off the bags. Counting only the 270, 285 and 300gr. bullets, 3 different mfgr's, the overall group was closer to 1 1/8" for all weights. The 350's shot a tiny bit low and the 235's a wee bit higher than the middle weights, doubling the group size.
: With 270gr.WW cast bullets & 23gr. 7625, (no filler) it held 1/2 to 3/4" groups at 100meters. reqiring only a 5MOA vertical sight adjustment for zeroing.