Sky,
Rust in a barrel eventually does a couple of things, one is to discolor the steel or after duration, the steel surface eventually deteroriates, pits perhaps is a term.
Years ago, I inquired on how to remove old bluing/rust from a Luger pistol. Brownells had some materials for such as you noted but application was key. I was told to dump the works in the vat of house house vinegar for a while, the old method. Amazingly worked fine and did not work on the metal more sorta speak; but inside and outside with my short prep.
Brownells has special plugs for guys that prefer to plug bores during rust bluing, fwiw. Old guys still use tapered wooden dowels.
One could ponder application of a penetrating oil, Kroil comes to mind, pour the bore full, and plug. I think a little better material for penetrating oil is a mixture of 50% acetone/50%ATF. ATF is one of the better oils at such that there is, fwiw.
Removal of rust that is below the surface line, does just that removes that controlling steel line. Uncontrolled abrasion is not my personal choice. Like the restoration guys, steel is usually either removed nicely or added on(welding etc), in barrel kinda hard huh. Lapping comes to mind, but is a real artform/costly. Reboring on an inexpensive rifle is probably not one's first choice either, 250 and up, and larger bore/size.
Old day, gunsmiths sleeved high powered rifles. The art is gone now, mainly because of the perceptions of pressure and liability.
If twas a Luger barrel, these are often sleeved for another round at life. Some early renditions(as well as current) will show the sleeve line fore and aft depending on the expertise of the doer. One of those sleeve lines can be removed from the picture with a sleeve machined with the threads/chamber on the 750. Front can be hidden with some cleverness at the lathe.
As the guys have said, tis remarkable what a "bad" bore will shoo like, especially in the older mil rifles..........just have to try it perhaps.
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