This is an interesting question. The answer is, I think your big bullet will stabilize. You have to know its length, but, basically the old Greenhill formula will apply reasonably well to most cast .458 bullets. They're in the right shape range, the right material range and the right velocity range from the 45-70 that it will still approximate OK. The free calculators (Google on "Greenhill formula"), say a bullet is OK in the 1895 up to a length of 1.6 inches, in round numbers. The 1895 has a 1:20 twist.