Mike is right. This is where you earn your Phd. in reloading. Plus there's a heck of a lot of fun in the process. I also agree with him that I don't believe you are doing anything wrong. It just takes time and persistence....that's the nature of old rifles in general and double rifles in particular.
I went back over your previous posts to see if you had an aversion to cast bullets. I did not see any mentioned. Is a gas checked hard cast bullet a possibility? They, cast, aren't quite so apt to increase pressure if a bit oversize as are jacketed. As you know your groove diameter, and shortly should know exactly, a mold can be had to fit the generally accepted .001 - .002 over groove diameter, provided it will chamber, and to the weight you wish, apparently 250-260 gr.. At the velocities you're talking cast isn't a problem. Just a thought.
