Hi Bob. I had an Army San Polo model 61. As it was an early rifle, it has (I was told) a Parker Hale barrel & has the 5-groove rifling normal to that model.
The Parker Hale Enfields have progressive rifling & thus are preferred to any repros that do not. The rifling is .008" deeper at the breech, than at the muzzle.
My Musketoon had this progressive rifling as well. I do not think the later Italian guns had this style of rifling. Mine shot splendidly with patched round balls.
I started off using .575" balls case from a Shilo Gang mould(4-cavity), however the bore was undersized at .574", which might be why PH sold them to the Italians.
Since a .575" ball would sit on top of the muzzle without a patch, there were a mite tight loading. After a while, I purchased a Lee .562" DC mould, which actually
cast .562"x.562" balls from both cavities. I used either the .020 ticking, or 10oz. denim I measured at .0225" or the .0235" railroad ticking. All shot well and the ball
must have slugged up as the fired patches were reusable.
75gr. to 85gr. 2F was my favoured loads for trail walks. My early testing with 75.0gr. GOEX 2F produced 1,308fps with the .575" pure lead ball and gave me a 5-shot group
of 3" at 100yards. I no longer have that target, unfortunately. 85gr. was my go-to load for this rifle as I found it actually shot better than the 75gr. load out to our longest
targets on the trail, 109yards(100meters).
In the 1970's I worked up loads for 2 PH rifles, a 3-band(m53) and a 2-band (m58). Both rifles shot well with 85gr. of 2F GOEX and .575" round balls and .022" denim patches.
Were I to get another, a 2-band 1858 would be my choice, however the 3-band with the 78" rifling would also be nice.
The 1861 makes for a better bush hunting rifle, though, and seems to give up nothing for speed of ball.