My 1-28 green mountain barrel in .50 caliber shoots the .430 diameter 300 grain XTP with a crushed rib sabot. This helps them load a little easier. As you can see, I push them pretty hard, but this is 50 yards with a 1X scope trying the two different projectiles. The first shot was the low 5 o'clock hit. After that, swabbing between shots, it would just lay them in time after time....



With the Nosler, you might want to get some HPH-12 and see if they do not load a little more solid. Also they are good conical shooters if you need to use big lead.

Where did you buy the barrel from? I think I would like to get one of them for my flintlock as well... Keep trying. Also my new barrel I scrubbed that thing really good with some J-B Bore Paste. I wanted all the rifling nice and clean...