I chose that example as a poster child to where even with a loading of smokeless to the same FPS numbers as black powder the "feel" from the gun will be different. I doubt the human body can tell the difference between a 1950's loading of 54r to a modern loading to the same specs as the surplus load. I guess that went a little over your head, sorry for that.Well, for starters, that's a dumb example because nobody here is talking about shooting "surplus" 45-70, which is unobtainable. But there are many shooters who do run BP through their vintage 45-70s, and that is a very authentic experience. As far as all the other reason that people seek out and shoot "modern" surplus ammo, most of the reasoning behind that has already been laid out.
I contend AGAIN that in shooting for example 54R, it can be 8mm, 3006, pick your poison, that if you load with modern components you could not tell the difference between the loads, now this also assumes you are using an appropriate powder for you given task, as in using 4895 for your garand and not 4227. To put it bluntly you can't tell the difference between modern and surplus if loaded to the same specs.