Designed by Remington in 1867, for chambering in its Rolling-Block rifle for the Spanish Government, this bottlenecked version of the 43 Spanish round predates the 45-70 by six years, making it one of the earliest of all modern cartridges using what we now consider to be conventional components. Two versions were originally designed, the tapered-case version was known as the Reformado and is somewhat less powerful. Ballistically, the bottlenecked version of the 43 Spanish was extremely similar to the original 45-70-405; it launched a bullet of slightly smaller diameter that weighed slightly less, a bit faster. The original load used a 0.439-inch diameter bullet of 396-grains with 77-grains of blackpowder for a muzzle velocity of about 1275 fps, startling performance for that era. Regarding bore size, much confusion exists in the literature. Actual bore and groove diameters are likely somewhat variable, as was common in that era. Carefully clean, oil, and slug your bore to find out what size bullet will best fit it. If a bullet about 0.001-inch larger seated in a case allows the case to chamber freely, accuracy and performance will be best. Obviously, the 43 Spanish will do anything the 45-70 blackpowder round will do. Modern practice is to keep pressures in original Rolling-Block guns on the modest side. I would suggest following that advice. A case full of blackpowder in this round launching a 400-grain bullet will likely generate about 25,000 psi. A charge of about 25-grains of Accurate 5744 should launch a 400-grain cast bullet of the correct diameter at about 1275 fps to duplicate original-load ballistics with significantly less pressure than the original load generated. Such a load is certainly adequate for deer and elk hunting at short range.