Firstly let me say that your "over thinking" the methods of getting accurate loads with your Pedersoli. A boolit that casts 0.4575" you are sizing to 0.459" when made of a hard 1:16 alloy???? I find that hard to believe! I shoot my 45/90 with an alloy carefully measured to 1:40 so that the boolit will obturate upon firing to completely seal the bore. I use a range of boolits and especially the "545gn PGT" (LHS of the pic) which is a custom lathe bored mold that casts 0.460" with a bore riding nose of 0.452" to align everything up at loading..
Your internal barrel measurements are all over the place, 0.4515"/0.458 at the breech and 0.450/0.458", with a 1:18" twist that is the Pedersoli standard and I have confirmed that measurement in all 4 of my rifles including my 1886 lever rifle in 45/70..
I use Federal large pistol, because the cup is softer ignition is more positive and fast. I did some prolonged testing some years ago, LP gave as little as 5fps variation while LR's were 40-55fps. Groups tightened from loose 2.5MOA to a consistant 1+ MOA average.
This group at 100yds is actually 11 shots because at the time I thought I had shot on the adjoining target! Load was 70gn Swiss 1 1/2 (2FG) with a custom 545gn PGT bullet.
Doing a bit of research on the great barrel makers, Harry Pope, George Schoynan, Paine, and the overwhelming factor is a tapered bore. All these great masters were keen on the idea that a barrel should be tapered for best accuracy. The old MH was tapered having .009 deep rifling for the first "4" and only .007 for the remainder; and even today, modern makers like Pedersoli have a tapered bore in their BPCR rifles. Parker Hale reproductions from the 1970's have tapered bores.
Many shooters think this allows them to in jam oversized projectiles for best accuracy, but from references in "The muzzleloading Caplock Rifle" and "The bullets Flight" by Mann, it would seem that the main reason was to accomodate the powder fouling where it mostly accumulates (immediately in front of the combustion chamber) so that second and subsequent rounds could be fired without cleaning (battlefield conditions)
In the history of the "Little Big Horn" massacre, we know that the trapdoors after a few rounds experienced jamming and lack of extraction. When I look at my own original trapdoor, behold no taper that I detect! Modern after market rifle barrel makers provide smooth parallel bores; so have we got it wrong?
Quote from The Pedersoli factory:
So........ given that we broach rifle our Pedersoli barrels and obtain straight lands and grooves with match grade tolerances we then add our final high quality feature which is to impart a very small taper on the rifling, from breech to muzzle!
Now when a bullet is fired in this barrel is is constantly entering slightly smaller dimensions and thus it maintains almost a perfect gas seal....resulting in those highly required "single digit" muzzle velocity variations as well as extremely small velocity spread (ES) in a long string of shots. With groove and bore dimensions held to the match grade standards of plus/minus .0002" and then having the little taper to the entire length of the rifling, we end up with barrels which are capable of accuracy equal to or superior to any other barrels made today, and at a price many can afford.
End Quote
So extrapolating from this data Pedersoli barrels run 0.450/0.458" at the muzzle to 0.4515/0.458" at the breech. Your cast bullet nose therefore should match the 0.452" for perfect alignment. You should just see where the lands have touched the boolit nose. All this explains why match shooting with a Pedersoli Sharps is a bloody science, NOT just another gun! nya:
How do I know all this? Comes from 40 years of shooting Pedersoli rifles including competing in 'World Creedmoor"