Kind Sir,
Just for the sake of discussion only...
You have posited some very good questions for the 45-70, etc., yet as far as I can see, unless I missed it buried in another post, you have not told us what brand of 'Sharps' you bought. Is it a Pedersoli, an IAB knock-off ala as seen posted and touted in the 'Cheap Sharps Club' or is it another variety as is available from other vendors or manufacturers?
Please Sir, It really helps to have all the data before any of us that shoot the Big Bores can proffer a definitive answer, and that is only fair to us and you as nobody wants to pass on bad loading data, just posting that you have bought a 'Sharps' does not really give us the basic info we need to help you out.
So, please tell us what the rifle 'is' and those of us that shoot 'it' may be able to offer more specific info. For example, your post about firing pin placement does not help us unless we know what type of rifle it is as the placement of pin impressions varies as you have probalby have discovered already.
I at present shoot a Pedersoli 45-70 made in the late '90's and the loads for it differ dramatically from a Pedersoli Sharps made in 2010, so please give that some consideration in your questions as those of us that enjoy the hobby would really like to help but need more data.Best regards and thanks for letting us offer our help!
'Ol Gabe