Corey, measuring the diameter of the Powerbelt skirts, or sabots that fit your bore, "might" give you a useful number to work with for deciding what diameter you need your conicals to be. But that plastic flexes and compresses a bit when you push them into the bore. So the measurement you get from them may not work.

What you really need to do is slug the bore with an oversize lead ball.

Clean the bore well and oil it. Then flatten a ball slightly so that it is slightly over bore size and has to be pounded into the bore, cutting its way into the rifling.

For an in-line, push it about half way down the bore from the muzzle end then push it back out from the breech end.

It's a little harder with a sidelock where you can't remove the breech plug. What I do is use a eight or ten inch section of steel or brass rod as an "inside the bore" hammer. The rod should be fairly close to bore diameter but thin enough to slide easily in the bore when wrapped with tape to protect the bore. Slide the rod into the bore. Then pound the ball into the bore about a quarter of the way down. Then by repeatedly turning the barrel up and down use the rod as a piston to knock the ball back out.

Measuring the ball will give you an accurate bore measurement of both the lands and grooves.