Well, I finally got hold of a caliper and found out that the bore on my New Frontier Beartooth is .496 on the lands and .506 in the grooves. This explains why only PowerBelts (.501 on the skirt) load well. The .44 cal. 300gr XTP's in a hornady green sabot for example, measure closer to .510.
The .475 Hawk bullet in the thin blue Harvester sabot measure .504 - slightly smaller than my bore, but still quite atight fit in the rifling.

How does this .496/.506 bore compare with other rifles out there?

BTW - if you're measuring projectiles and find something that looks like it will work well for me, please let me know.

IM jaybe