If your looking at the stepped down diameter at the muzzle end thats about 1/2" long I would only speculate. Possibly a major thread dia for a threaded suppressor just a guess or a threaded muzzle brake. I havent looked at the diameter size but it could be a major thread dia. I check this evening when Im out in the shop.
Then there is the stepped down diameter that seems to be about 6" or so in lenth stepping down to fit the front sight tube diameter. I think Ruger may have down it a little differently in my opinion.
Then there is the turning marks which is typical of a lathe cut. Barrels that have a smooth, no tool marks surface can be the result of a centerless grind operation or a very light finish turning op then a coining operation which on a blued barrel is really nice looking. One of my Beretta over and unders have a coined finish and it real nice to look at. As smooth as a ground surface but less glassy than a polished blued barrel.
I think the barrel finish on Rugers Stainless barrels are a production lathe turned, lower the mfging cost, get it to market approach to their manufacturing. I dont know if the blue barrels have the same typical tool marks. Maybe someone might comment. Ive never really looked at the blue job on 14's.