I like your choice of cartridge.The 30-40 Krag is my favorite.Some years back my family used to shoot with a man who built a double rifle 45-70 on an L.C.Smith that my father got for him.He owned a state of the art machine shop and was one of the sharpest men I've ever known.After spending many hours of work,it was ready to be proof-tested.This was back in the day before the Destry proof-house came into being but the testing procedure may have been simular.This man fastened this L.C. rifle to some straw bales as I recall and ran two long strings to a safe location to pull the triggers.I well remember the results of the test.Both sides of the gun were destoyed by the firing pins blowing back thru the action and stock.The rotary bolt was intact.There is very little to hold the firing pins from doing this on Smith guns.The pressure of an old 45-70 should be well below that of a 30-30 or much less than that of the 30-40 Krag.My father and I often thought about the loss of that fine Smith gun .I would not concider building a rifle on a Smith.I enjoy shooting them as the fine shotgun that they are.Hope that this can be of some help.Charlie.