Not sure at what point a late reply becomes bumping an old thread, but I would like to be kept in the loop for how this project progresses as well. It sounds like here in Australia we're slightly luckier with martini pricing. (Should be thankful for small mercies I guess!)
Martini conversion is popular here. Although the .310s have a small following, there are more people with .310s without cartridges than cartridges without .310's... if that makes sense. ...and a whole heap with conversions.
I own three at present, including the oft-discussed .44 rem mag version (middle) a .22 "K" Hornet (top) and .22lr (bottom, obviously...)
All were 'bubba'd' looong before I got my hands on them... if it makes a difference...
I paid between $125 and $150 for each of them.
The history of these things can become convoluted to chase, but two are marked 'BSA' and one: 'WW Greener". Some have a kangaroo stamped on the top of the action, and some don't. Here in Australia, it seems like a lot of the 'converted' ones passed through the hands of 'Sportco', and are known as "Sportco Martinis".
Two of Sportco's favorite tricks were to rebarrel to 22 Hornet, by affixing a .223" (rimfire) barrel with a 1 in 16" left hand twist and reaming to hornet, and, converting to .22lr by fitting the same barrel and grinding down the 'horns' of the closing lever in such a way that the 'closed' position of the breech-block aligns the standard 'center-fire' firing pin with the lower rim of the .22lr cartridge case. Thus it is 'theoretically' possible to return a converted action to centre-fire by either building up the horns, or fitting a non-modified lever.
Unfortunately this second system leaves the block not lining up with the side walls of the action body which is a little unsightly after it's been brought to your attention... I ground the sidewalls down to be flush with the top of the block, so it's probably too late for me...
As a side note, my good mate 'Bruce' (but we call him 'Bluey', on account of his red hair) assured me that the left hand twist was an absolute necessity for making a northern hemisphere rifle shoot worth a damn in the southern hemisphere... something to do with the coriolis effect going the wrong way making the little boolits dizzy and all...
Anyhow, for what it's worth to those that follow, the .44 mag is intact, and beautiful, after 5+ boxes of full power factory loads. (240 grain jacketed at 1740fps) There are three basic camps;
One camp says: "Why risk it?".
Another camp counters with: (Insert famous name here) performed destructive testing and failed to blow one at 65,000psi (this I believe might be true but the name escapes me for quotation purposes... it was one of the "Ackleys" of the world however.)
The third camp (my gunsmith included) will happily tell you which maker made the strongest actions and only convert those ones.
I shoot mine happily, and yet, if I did it again I would opt for an ever so slightly smaller case, either .32-20 for plinking or .357 magnum for hunting. I suppose the very fact that THAT is in the back of my mind puts me in the first camp...