[W. C. Quantrill]

It appears that your Martini has been "customized"?

Rebarreled?

For sure .50-70? It appears to be an unknown quantity there. I load .50-70 for my buffalo gun. BP in cartridges works better if it is compressed. You need to do a couple things here. One, get some bullets. Since your rifle is bastardized, you dont know for sure what the bore is until you slug it. Get a dead soft fishing sinker, egg shaped, just bigger than the bore and drive it in the end of the barrel then push it through and let it drop out the chamber end on a towel. Mike it and see what your barrel measures. That is the only way to know if you are going to need .500, .505, .510, .512, or .515 bullets. That is going to determine whether you can use an off the shelf loading die or whether you will need to get custom expanders.

Fill a case level full of powder and then measure what you have. A case full should be right at 70 grains volume. Weigh it now. Now you have some reference point. I would back it back to 65 grains, add a .030 over powder card, and seat the bullet. Before seating, I would measure the bullet for how deep it will be seated, then use a compression die to seat the OP card to that depth then seat the bullet easy on top of it so as not to deform the bullet.

All you need for the .30-Carbine is some .308, 110 gr cast bullets. Obviously, there isnt any BP loading info for the .30-Carbine. I would use FFFg powder, dip the case level full then seat the bullet--compressing the powder.