Bowtech7Just some thoughts.... stay away from the Pryo's if you can... T7 and I would suggest T7-3f for a cap gun would bethe powder that I would suggest... in fact either a 100 grains or 110 grains which ever you feel more comfortable with.. I would also suggest that you use a #11
MAG cap or even better a RWS 1075+ Dynamit Noble cap.
If I were to suggest a bullet to you and it would be the one I am using it would be the Lehigh .458-305 in either a MMP HPH .45x50 sabot or a HPH24 45x50 sabot which ever one fit more snug but still gives you the ability to load easy enough. This bullet is a real 45-70 rifle bullet and is held to tighter specifications than other ML bullets.
This is what the bullet looks like...

It is a long bullet and a 1-28 twist is right at the max you can shoot it with. The sabot has to be tighter so that the bullet gets every bit of rotation the 1-28 will give it.
A really good runnier up is the Knight Bloodline (built by Lehigh) .458-300. It is a shorter bullet and stabilizes very well in a 1-28 twist.

Then probably one of the best production bullets is the Speer .452-300 grain Deep Curl - but they are really hard to find - Speer can not make enough of them. They are a bonded copper/lead with controlled expansion.
But for elk and were it me... I am using the brass bullets...