Paul has a good question. 1st of all, the gun has a custom barrel on it. If it is a GM barrel, it could be a 24" twist, or a 56" twist. I am not familiar with other 50 cal barels, but have heard of 22" up to 15" twists.

If it is a 56" twist, it will probably shoot poorly with 300gr. and even more poorly with anything heavier. If it has a 24" twist or faster, it will be exceptionally accurate with bullets to excess of 600gr.

I agree with Curl, it will at leasy duplicate the .500 NE.

My .50 Alaskan Rolling block made 2,000fps with 500gr. in a 2.1" case. The longer case of the .50/110 will be better yet.

Curl's suggestion is a good one as to initial load - but - measure the rifling first. With slow original-type rifling twists, the 300gr. would be a plains-type game rifle only out to about 300yards max. - if you can hit them, that is.