Tom:
Can't help with actual strength data, but generally the Martini action is stronger than needed for all the usual chamberings. The GP-type shotgun action is relieved quite a bit behind the main pin (above the wrist), probably to save weight, so for high-pressure cartridges, or big cases with lots of back-thrust, the Martini-Henry or Martini-Enfield rifle actions are to be preferred. I believe they have the same or similar breech width, and action side-wall thickness, but will measure a couple when next I have time. Fine examples of the Martini-Henry, Martini-Enfield, Martini Cadet, and Greener GP reside in my gunsafe.
In Britain almost a century ago, Martini-Enfield rifles were routinely rechambered for the .375 Flanged Magnum, probably the largest practical cartridge for a sporting rifle on that action.
