The original 1873 bullet weight was 405 grs. It was loaded with 70 grs. of black, and later, 55 grs. for carbine armed cavalry. In 1879, the army began testing the 500 gr. bullet load and it was adopted for use in August, 1882.
CZVZ, I'm 51 and have been shooting the beloved & grand 'ol 45-70 for years. As others have suggested, keep loads light and you will enjoy it more afield. I load mine these days with a 405 gr. bullet at original rifle velocity levels ( which was 1330 fps ) . They are reasonably mild from an 8 lb. rifle, very accurate, and make quick whitetail stoppers. I have harvested 7 deer with a 45-70, the last being a buck last month. One does not need the punishing, thermo-nuclear loads you read & hear about being loaded for 45-70 to be effective in the deer hunting arena . If interested, email me for load recipes. Best of luck.