I'll push a slug all the way through the bore to feel if I can tell how uneven a bore is, but I admit I doubt I'm much good at picking up small differences. The only bore I worry about slugging is just past the throat, then push it back out the way it came in.

A good rifle barrel can have some choke in it, sort of like a shotgun barrel, and I believe it's a mistake to judge a barrel or choose a bullet based on what could be a little tight spot at the muzzle. It's very possible that a good cut rifler is aware of that, particularly since a prefinished barrel can't be shortened to clean up a bad muzzle.

Some barrels just don't shoot well, but that doesn't seem to be the problem here.