Inline with removable breech plug? Or the new Rice percussion fast twist 4140CM barrel?
You can remove the barrel from the inline, and then the breech plug from the barrel. Then, take a larger than bore diameter ball (.54 cal), or bullet (.52-.54 cal), and using a 7/16" diameter piece of solid brass bar stock & a 24oz dead blow hammer, drive the oversized lead projectile through the bore from the breech end of the barrel to slug the barrel. This will create an EXACT REPLICA of the lands & grooves pattern of YOUR BORE. Make sure the slug drops out onto something soft like a bunched up bath towel.
The best way to obtain the bore dimensions of the sidelock barrel from Rice, would be to remove the breech plug, and to perform the same task as for the inline.
Second best is to use an old time gunsmithing material called Brownells CerroSafe, to create a cast of the bore. This is done by stuffing a rag down into the bore to block the casting material from flowing any further down into the barrel than you want it to. Then preparing the CerroSafe, and pouring the cast. After it cools, and drys, then removing both the cast, which can be measured, and the rag.
Third best is to take an oversized lead ball, carefully hammer it into a still oversized oblong shape, drill a very small, undersized pilot hole into one end of the oblong, thread a very tight fitting sheet metal screw into the pilot hole, and VERY, VERY CAREFULLY hammer the lead slug into the breeched barrel's bore by tapping on the sheet metal screw until the slug is far enough inside the muzzle of the barrel to have acquired an impression of the rifling.
Pulling the slug out of the barrel without either stripping the screw from the lead slug, or ruining/marring/pulling the lead out of dimensions is tricky, and requires both luck & patience.
The end result of any method of bore measurement is to ACCURATELY DETERMINE the EXACT depth of the grooves, as well as the EXACT bore diameter.
With those two measurements, you are able to purchase factory made bullets that hopefully shoot accurately from your barrel; or start casting your own bullets, which will require a set of fixed diameter sizing dies, or either a Hankins, or a Swinglock adjustable sizing die.
Hope this helps.