Starline carries both .50 Alaskan and .50-110. The 110 gives a little more room to boogie. The .50-110 can drive a 525gr bullet in a modern 1886 winchester to a bit over 2000fps (2100 comes to mind). Not sure in a martini, but i assume 500's at 1800? Or a 300gr @ 2200fps like the factory loads.

https://www.starlinebrass.com/brass-cases/50-Alaskan-Brass/

https://www.starlinebrass.com/brass-cases/50-110-Win-Brass/