I have just purchased a 45-70 BFR and i have to reload this thing or i cant afford to shoot it.... do i have to keep my formulas consistant w/ the trapdoor guns to keep it from flying apart? I'm interested in expanding on the 45-70's potential for greater distance in some loads (hot 300 gr'ers) and more knock-down in others (500 gr'ers) but there is little or no realoading data available for the BFR. What experience do you have with this? do you reload your BFR 45-70?
I reload all my 45-70. The gunsmith at Magnum Research quoted 31,000 PSI as maximum pressure for the BFR. The corresponding value for CUP is around 32000. That's more than a trapdoor (28000 CUP), but less than a Marlin 1895G at 40,000 CUP. A load that is rated no more than 30,000 CUP should be fine.
One problem you find with the 45-70 is keeping the powder near the primer. The recommended approach is to purchase some cotton batting from the local sewing materials store. A small patch placed on top of the powder will help ensure consistent ignition.
There is a nice article on PSI vs CUP here. You can find a couple trapdoor loads using Hodgdon powder here
If you are interested, Magnum Research will fit the 450 Marlin cylinder to your frame for $150. That's a hotter belted magnum load. Two cartridges using the same frame/barrel.