Muley, the PRB and that .54 1:70 twist is the perfect combination. And no doubt effective as per your end result. But I have a question. Did you ever actually track that cow, or just go up to her? Hunting the east, particularly in heavy laurel or swamps is a whole different world than hunting the aspen forest or sage flats. (And I've done both). In some places in these areas, if an animal goes 10 yds it may disappear. You pretty much need a blood trail to follow. Even in the mor open forested areas when the leaves start to fall, the red leaves of the sweet gum and red maple can make it hard to pick up minute specks of blood.
A case in point. Quite a few years ago I jumped 3 deer from their beds while hunting on a side hill. Staying above them I was able to get ahead of them and as I peeked over a rock outcropping they were about 50 or so yds below watching their backtrail. I picked the biggest one and dropped the flint down on the frizzen. At the shot all 3 bolted and I was able to watch them and saw 'my' deer fall. But rather than go to where she laid, I went to where she was when I shot. There was snow on the ground so tracking wasn't an issud. But from start to her demise (about 60 yds) there wasn't a single drop of blood and no leg was being dragged along. Every indication from that PRB was that is was a complete miss. Only I knew better. From that day on, I always follow each and every trail after every shot for a minimum of 100 yds.