Wendy,
I you want to have a repeating load, you’ll need to pick a bullet that the Henry Big Boy will cycle well. I had a .44 Big Boy for a year before I gave it to my brother (he absolutely loved the thing).
My .44 Big Boy was very sensitive to bullet length and profile. It would not load longer Keith style bullets, nor the longer Lyman Devastator hollow-points. I finally settled on an LBT 260 WFNGC and a MP 433-640 for my cast bullets of choice. It seems to me that the Big Boy was designed to shoot jacketed hollow-points, and other profiles may not feed as desired.
That being stated, I found the Big Boy to be extremely accurate, even with the 225gr Barnes solid copper bullets necessary to hunt with in California.
One of my favorite loads is using W231 and the MP 433-640 bullet. I expect it would chronograph about 1100 fps, which is plenty fast enough to kill a deer at 50 yards. You should be able to easily replicate a load like that with Unique.
I hope this helps,
Surfdog