http://www.serbu.com/rn-50-break-action-50-bmg.html
New entry. No idea how well they hold up, but I remember the Hesse .50BMG well enough not to risk it.
I seem to recall a big bore along the lines of a .700 JDJ or similar being made on NEF break-actions. I'll have to poke around and see if I can find the specs on it. Again, no guarantee that they survive beyond the 100 rounds I estimate most would shoot them.
Edit: found it, 585HE. 720gr lead@2700FPS, HR Topper is among the common actions, estimating 65,000PSI according to the manufacturer.
Count me in the camp somewhat disappointed that Henry just making break-actions. I keep drooling over the Chiappa and Uberti repro rifles, might have to start saving to buy myself a graduation present.