How strong are these actions? Looking at maybe. 577 ne in that action. Probably stick to my cz project but I like to keep a open mind

A full nitro 577? Not only no but Heck No!

For their age, they are very strong, and bend rather than blow up if you overload them. Cannot see you getting such a long round into the breech though - rounds have to be quite short or well bottlenecked to fit in the curved feed ranp.

Put a new barrel on one years ago and loaded up the original MH to 500grn bullet at 2100fps. Was fun, but the cases didn't last (two shots)

The local lads who wanted to shoot their Martini's alot with easy loading rechambered to .45-70. The rifles will easily take Marlin type loads but they are not Ruger No 1's and have seen two bent with overloads.

Have two in .303 which of course are easy to rechamber to .45-70- but I like .303. One is still factory original (1896 conversion- built originally 1876) and onother that I put a new .303 barrel on (also originally 1876 and factory re-barreled in 1902- but barrel was completely gone.

You can use a Greener Martini of 12ga fame for more power.
Handled hot loaded 3.25" 12ga brass cases fine.,
Have one done soon in my 585HE. Ed

I was curious . Not that I plan on buying one as I'm going to get me a cz550 built by ahr in the foreseeable future.

I built one on a Greener Martini, and the cartridge slipped in like it was designed for it. I got the idea from an article in one of the Gun Digest's which showed a picture of a British built Martini in the full 577x3". I sold the gun to a fellow forum member when other projects were of more interest (and cheaper to feed!). He may speak up and let you know more about it, as I remember him posting a pic of it after he worked on it.

Mike, I'm the one who bought that rifle from you. It now lives with a Martini enthusiast in Cut Bank, Montana. I fired it just a time or two with the Woodleigh 650-grain soft nose in Jamison brass over a compressed load of black powder. Really quite pleasant! I sure wouldn't have tried that with smokeless though.

Thanks for the update Bill. Always wondered what you ended up doing with it.