Okay, I'm back.

It doesn't fit the Martini-Henry. Well at least a 557 2 2/3" NE with a 650 gr. cast lead bullet doesn't fit. So of course a 3" won't either.

The rim binds in the rear of the breech block.

Now someone is going say you can grind the load trough in the breech block to get the cartridge to feed.

The breech block of the Martini is the "Locking lugs" of the design. This knuckle rests in the knuckle seat at the rear of the action.

The knuckle bears the load of the cartridge on firing. The knuckle attaches to the rear of the block adjacent to the load trough.

Any deepening of the trough that is done for the 577 NE to fit has to be done in the rear of the trough adjacent to the knuckle, effectively weakening it. Looking at this, it appears to be a modification that "will" weaken, not "may " weaken.

I also checked a Francotte large action and it won't work either.

What will work is a Greener Martini. I have a 577 2 3/4" NE built on that action. Works just fine.