arkypete: I have four K-31 Swiss straight-pull military surplus rifles; all have excellent bores. According to the people on the Straight Pull Forum at the Parallax site, their rifles were inspected every year. If the barrel did not meet inspection, it was replaced at the individual soldier's expense . This is one of the reasons given why the bores of the Swiss milsurp rifles are usually good. (Other reasons are that they didn't use corrosive ammo and they were not used in WWII.)
Brady

When testing loads, I've seen several of my rifles pick up about 100 fps and stay there afer 4-5 shots through a clean clean barrel. Mine did just like yours, increasing slowly with each shot fired. I don't clean mine overly often, when hunting I never clean it.

I am not sure if it is geramin but, the Swiss ball has a grease or wax band around the bullet. That is very unconventional for jacketed ammo. The stuff shoots very clean and the bore cleans up in a jiffy. Maybe the wax has something to do with that. You are basically always shooting a bore with a bit of lube, maybe like an oily bore all the time? Anyway, it is also the most accurate surplus ammo I have ever shot. It is priced accordingly.