Are the Mark X actions in a standard and magnum size or are they all a standard size and then opened to handle the larger cartridges? If there is a magnum size action, what calibers came from the factory in that action? I would like to buy a Mark X but I want to be sure I get the largest action possible for any futher projects I might dream up.

They are all std size Mauser actions that are openned up. and on top to that they are openned up in front and that is a no no according to all the Custom gun experts, yet I have never heard of a MkX blowing up or even developing headspace...not to deny that there are folks out there that could blow up a tank with an improper handload...

I suppose, at least in part, that its mostly modern metalurgy that makes them reliable and safe..

As Ray pointed out, they are all standard size. The ones in .375 H&H have been opened to the front, but I have never heard of one developing troubles with standard loads.

If you are considering a cartridge larger than the H&H, such as a .458 Lott, it is more prudent to start with an action other than the .375 and then open it up to the rear.

I had one of the Mark X rifles in .375 H&H. Nicely accurate, but as has been said, they open them up in the front. I got rid of mine because it scratched the hell out of my brass with what I felt was very deep gouges. If you run into either a Mark X Alaskan (the one I had) or the Whitworth, stick you finger into the action and feel the sharp ragged edge where the lower locking lug recess is located. I've check out two Whitworths and like the Alaskan I had, it feels like saw teeth.
I've given thought to try and see if they could be polished out or smoothed up in some way, but haven't quite figgured out how I'd do it. Otherwise, they're pretty nice reasonably accurate rifles.
Paul B.