First, I am not going to go into any theoretical discussion on the relative strength of the Martini actions. I am not an engineer. I am not a aware of any analytical testing done when these designs were created. So I can not quote any mathematical answers to your question.

I don't put much stock in the CIP system of proof to give us an answer as it does not give us a true measure of strength of a given action. Rather it gives us a measure of an action to contain a given chambering. It may be no where near to maximum values for the action. Further the CIP proof charges are only 120% of peak working pressures. The American proof system test the design from 140% to 180% of peak working pressure depending on chambering. The test piece is retained as an archival reference in the American system.

I am aware of some later individual tests of these action to failure similar to Ackley's test of post WWII actions. But just like Ackley's test, the resulting wrecked actions don't really transmit to hard numbers on strength, since pressures involved are unknown. It does give a subjective idea of strength.

I stick with what I have been taught and what I have experienced, and what I have learned from others-credible others. People like Chick Donnelly, Bob Snapp, Frank De Haas and P.O. Ackley. I knew Donnelly well and lived a few miles from him. I talked with Ackley a number of times at Donnelly's shop. I corresponded with Bob Snapp a number of times. I corresponded with De Haas by letter and telephone a good deal during Gunsmithing school. I listened to these most wise men and learned.

Where no data exists, I follow the guidance of these men, I try to stay with in the range of historical chamberings and similiar cartridges.

The .225 Winchester is a chambering that is argued as safe/unsafe in the the Cadet. I mentioned bulged chambers in this cartridge. I am relying on my memory of work I did in the mid to late 80's. I remember two of these for sure and possibly a third. I discussed with the customers the issues expressed by others about this chambering being to large for the tenons.

After shooting these rifle for a bit they were all returned with brass sticking in chamber-determined to be from bulged chambers. I of course looked these actions over and checked the dimensions from the original build ticket and since all were unchanged, othe than the barrel, all were rechambered, .219 Donaldson Wasp, with never another problem.

Bob Snapp has done a number of these in 7-30 Waters and when I asked about that, he reported no issues or problems. In fact Bob told me he built one of these for himself and took it Caribou hunting. Bob did most of his work on the 4A action, the thinwall cadet. He told me he thought the thick side was big and ugly, and did not do much with them.

When you ask about the merits of the Francotte action, you do need to be clear about which you are talking about...Saying "Francotte Action" is like saying Winchester Lever action. While I assume you are referring to the Francotte style actions found in the various version of the BSA single shot, Francotte and various others did make this action also. I would have no qualms working with the Francotte made guns, and most of the Belgium made using the Francotte patent. There are others I would be reluctant to deal with if I were still doing this work.

The other small Martini's I would not do anything that was greater than the original chambering.