When i was a kid my dad agreed to take a coupl eof old military rifles to clean up for someone. Then the guy brought them over. All of them looked like they had been stored in a shed outside or something :O As far as i remember, there was an enfield, a garand, a springfield, and a k98, all rusted so badly that all the actions were totally unmoveable, thick coat of orange flaky rust on the outside of all metal. My dad said "You can try to clean those up, but I'm not touching them!" So i tried, and over the course of about a month, succeeded
First thing i did was spray the 4 guns down with penetrating oil. While that was working in, I used coarse steel wool and more of the same oil to scrub most of the rust from the outsides. Repeated every day for abouot a week, and i was finally able to knock the bolts loose and remove them to soak in yet more penetrating oil. After that, disassembled what i could, and used fine steel wool, a bronze "toothbrush" and lots more oil to clean more. Then switched to flitz and managed to get most of the pitting out. Scrubbed the bores with oil and bronze brushes, then a patch with more flitz to get em shiny again. Not sure if they ever turned out to be shooters, but they all worked smoothly again, and dry firing testing showed that they did in fact work in that respect. With the amount of crap removed from the bores, rifling was definitely damaged. I remember taking the enfield out to the range and trying it, and found that it grouped about 18 inches at 50 yards with handloads if that's any indication.
If i had these guns now, i would probably re blue since all metal parts were scrubbed down to bare metal as there was 100% heavy rust coverage. Keep them for decorative purposes, kinda to say "Yeah, i've got a garand..." but not really worth shooting imo. You might be lucky and end up with a decent working shootable gun. You'll have to do all the work to try it out though

Look at it as a labour of love, and experience in cleaning/refinishing guns
One thing you might want to think about is, is the gun registered? You might want to call and find out/get it done to cover your arse just in case. If it's really badly siezed shut, does it count as deactivated?
