I've never heard of anyone building a 458WM on a long action BLR , but more than a few BAR's have been converted . I shot one that an old friend , now long gone , had built in the early 1980's . To be honest , it wasn't to bad to shoot . Being gas operated help spread the recoil forces out over a longer time period , but it still had some serious recoil . The BLR and the BAR have very similar bolt designs so it is possible , but like you said , it would be a bit of a beast .