A little "Bark" from me here..

In the old catalog from H&H( 1887-1890), they advertize for a 12bore "xtra powerfull" loaded with 9 drams of black. I believe that load should/would/ought to be a 3.25" case in order to contain 9 drams. The weight of this rifle is between 14.5-15Ibs.
In the same catalog , H&H also advertize for 10 bore, shooting 9-10 drams black and a an 8bore shooting 10-12drams black. All of them using round ball or short bluffheaded cone bullet.
I believe from looking at various of old catalogs, gunbooks etc the "normal" 10 bore paradox was a 2 7/8"(70mm) case, shooting a 875grain bullet @ 1550 Ft/Sec with 8 drams of black.
The 3.25" inch shell for doublerifles in 12 and 10 was not introduced until 1887-88, and the H&H gunmakers was apparently the first to offer them.By 1906 catalog its all gone.
Tolley was maybe the last to still offer fully rifled 8-10bore up 1st WW in their standard lineup.
My Manton, like this H&H 10bore appears to have been made by WC Scott. My asumption for this, is that on all the hammers doublesrifles, backaction, jones underlever, I have studied as so far, books, internet etc has the same profiling, of hammers and pecussion fences... very identical with those made by WC Scott. I can be wrong though, but everything points that way...so far.

Sir William Gordon Cumming stats after consulting Sir Samuel baker in England, that Baker recommend 10 bores carrying 12 drams of black.

My manton does 1745Ft/sec with 900 grain teflon wrapped bullet which translates over 6000 Ft/Ibs.
I believe even the old 8 bore with 1460 grain bullet loaded with 10-12 drams of black could reach the 600 NitroEX in striking and smashingpower.