I jumped in on this one a little late ... I was amazed at these bullets' characteristics, having missed the source that you said, and thinking they came from American Civil War sites ... with those Broad Arrow markings! (Bullets marked like these - made under/by British Government facilities (likely Woolwich) - would not have been sold to the US/CS).

The most common muzzle-loading Enfield model was the long (3-band) Pattern 1853 (there was no P1859 (in .577 caliber), nor any long (3-band) P58). There were also shorter 2-band models (P56/58/60/61) that fired the same ammunition.

The first P53 bullets were .568-in in diameter, some with boxwood plugs, some without. The size was changed about 1861 to .550-in, as the .568 bullet was found too hard to load after the bore became fouled. These were all wrapped in paper from the cartridge, with a lubricant on the outside of the paper (meant to be loaded with the bullet). The boxwood plug was later changed to one made of baked clay. The plugs were designed to make the bullets expand fully (apparently, this was necessary with the .550 bullet).

Since the bullet in the photo appears to have been fired, it would likely mic out to about .575 or so, having expanded to fit the .577 rifling (with the paper still around the bullet). If you find some unfired ones, they should mic at .550 or .568, as Kevin stated.

The iron plugs are somewhat of an oddity. The bullet fired from the P1851 Minie Rifle had an iron cup, but this was a much larger bullet (about .70 caliber). I haven't heard of an actual iron plug before.

Very interesting to hear about these bullets - thanks! Mostly we only see bullets from WBTS sites.

Geoff Walden