The Barnes Original 300 grain FNSP is a pretty stout bullet. They expand very little on deer. I don't know who you heard say the Speer 300 grain UniCor destroys a lot of meat, but from what I've seen, of the Speer, Hornady, Sierra and Remington 300 grainers it is probably the least damaging as far as overexpansion and fragmentation goes. You probably cannot find a much better bullet for deer in the 45-70 than the Speer 300 grain UniCor. You should stick with it and kill something with it before you pass judgement. That said, any of the Speer, Hornady, Sierra or Remington .458" 300 grainers are good for deer when pushed appropriately.
