I have H&R single shots in both 500 S&W and 45-70. Some things to note:
- at the same pressure, the 45-70 has it all over the 500 S&W for every bullet weight. Yes, you can run 45-70 at the same pressure as 500 S&W in a Ruger #1 or H&R, and the brass will handle it;
- bullets for the 500 S&W are 0.501", not the "standard" 0.512", and so the selection is much more limited than for 45 cal;
- component costs are much higher for 500 S&W; and
- it's a very simple matter in a single shot to turn the 45-70 into a 45-90, 45-100, even a 45-120. Not so for the 500 S&W - a 500 Alaskan or 50-90 has a different bore diameter and the rim diameter will pose a problem as well.
None of those were reasons to stop me, nor should they you, but are good to know.