GregSY,

The 70 in .45-70 refers to 70 grs of black powder.
The original arsenal load from 1873 had a 405gr hollow base bullet bullet with 70 grs of black powder compressed into a nearly solid mass. Carbine loads later used the same bullet with about 55grs of powder. About 1881 the rifle load was changed to a 500gr bullet, still with 70grs of black powder.

Jerry Liles