Larry,
If there had been a big diference between the four varieties of J powder, it would have been marked on the flats of the gun which variety the gun was proofed with.

It wasn't. End of story.

We pay attention to modern loads in old guns because we like our fingers and eyes right where they are. The option, I suppose, is using something modern, but, even those fail, occasionally-I'm not chasing any K guns at the moment.

French proof, even with the old powders, was nothing to sneer at. If my Halifax had been proofed with powder J (it wasn't) I'd likely still feed it the 2 3/4" low noise and recoil Winchester loads. The wall thickness is a bit off the charts with that old gun, as it is on many old French guns, and I don't worry much about it letting go, but, it is over 100 years old, and a pound of prevention is worth a ton of cure, in this case.

I'd bet they exist, but, in the years I have been paying attention, I haven't seen a good photo of a blown up French gun. It doesn't seem to be a huge problem.
Powder J, M, S, or T.


Best,
Ted