Is a modern replica of the 1874 Sharps suitable for .45/70 loads for the 1895 Marlin, etc. or should the reloads be limited to 1873 Trapdoor power levels? Good loads? I have 300 gr lead and jacketed bullets, and 405 gr lead and jacketed bullets available.

I believe that only the Shiloh Sharps is stout enough for those loads. Pedersoli, for example, advises that 27,000 PSI is the max recommended for their rifles and I would imagine that is a good guideline to use, in general.

To wring the most accuracy out of a Sharps there is really nothing like blackpowder. I've tried smokeless and the results just aren't the same. Just doesn't "feel right" either.

Often, 28,000 CUP is cited as the MAX for the trapdoor. I believe that is the max level that Pedersoli recommends. Therefor it is Trapdoor level loads that you are "limited" to. However, you can move 300 gr. jacketed to 2000 fps and 405 to 1700 fps within those parameters. That is pretty good performance.

Tap:
When I had my C. Sharps .45-70 I used smokeless for moose hunting. My loads were all for cast bullets, never would I allow a jacketed, factory bullet to be fired thru my rifle - sacrilege. My loads were 47 gr.s of IMR-3031 with the RCBS 405 FNGC bullet and 43 grs. of IMR-3031 with the RCBS 520 gr paper patch. Yup! I used their .45 cal. PP bullet with smokeless. I fooled around a bit with different bullet hardness & for hunting, I wouldn't go harder than about 30:1 lead/tin. I fired up to about 52 grs. of 3031 with the flat nose bullet with no problems but accuracy fell off & recoil was more than I liked so I stuck with the 47 gr. charge. I'm not saying these loads are safe in your rifle, only that the above is what I did. I found I liked the large paper patch bullet for moose better than the flat nose. Seemed to give more penetration. Hope this helps. Bear in Fairbanks