Hi Pete,
I had a similar experience with a tight bore but in a new barrel...I tried to slug it with this Aguila SS 60 gr. bullet and it was so tight that I needed to push very hard and didn't get the feel, the slug actually got semi-stock in the last part of the barrel...I pushed it backwards, bore scoped it and it seemed OK.
I thought that I have cleaned the barrel but it was not the case, I was making 3 barrels and got confused....So I cleaned it thoroughly and found that it was full of a gummy-sticky black stuff that took me many wet & dry patches to get it out...Then I slugged it again but this time I lubed barrel and slug (regular lead bullet lube).
The slug went through the entire barrel nicely and fast enough so I could feel the barrel getting tighter to the crown area...I slugged it about 4-5 times but to tell you the truth, I ended up cutting the crown at at the tightest spot and EXACTLY where it was marked by the manufacturer...This barrel had no tight-Loose-Tight spots anywhere and this is all I got from slugging it.
If the manufacturer marks the tightest spot, they are always right on the money (my experience) and all you need to do is to slug it and measure the breech side for determining the chamber dimensions that you want to cut...
A friend from WA gave me a BR barrel that was not shooting good anymore...I bore scoped it and found what you found: Grayish butter like spots...
Then the lead had an amorphous shadow but no visible pitting that never came off and showed no other signs...Since the barrel was 24.5" and from a very good family, I decided to play with it and to start from scratch
I cut it to 22", re-crowned, & re-chambered it...Then I slugged it and discovered that this barrel had no choke, or at least I didn't feel it and after measuring the slugs taken from just ahead of the chamber and comparing them to the slugs
at the crown, they measured about the same (+_ .0005 or maybe .001) which is a negligible difference in my perspective but some other people may consider .0005 to .001 a choke.
I had no use for a re-worked 22" (plus tenon) barrel but then I put it on a CZ BRNO 452 without any back boring for tuning...As is it shoots good for what it is, similar to my Anschutz 1710 but not anywhere near a well tuned sporter.
Regards,
azuaro