Greetings fw,
Bench gun rifles are made for both round ball and slugs(projectiles) They look alike on the outside, but the barrel is rifled for a patched round ball or a slug. I do not know of any barrels that are made to shoot both the PRB and the slug.
My Ken Briesen round ball bench rifle 58 caliber and uses a .575 ball, .018 patching, and 180 grains of 2ff black powder.
At the NMLRA nationals and the TMLRA state matches here in my home state of Texas, the course of fire is shot at 50, 100, 200 yards, There is no maximum weight restriction (mine weighs 51 pounds) and only iron sights can be used. Some matches require open sights only, while other matches allow the use of AM (ANY METALLIC) sights.
slug guns matches enclude a 300 yard event, and telescopic sights are allowed for the 300 yard event.
The actions are usually underhammer design with a stainless steel sealed ignitor system set in the barrel. The actions are designed to accept a PRB or slug gun barrel, or both.
The NMLRA has a seperate course of fire for offhand Schutzen rifles using a slug or conical projectile.
if anybody is interested, the TMLRA is having it's annual state championships at Brady, Texas June 7-12, 2005.
Hope this will answer your quetions.
Best regards,
John L. Hinnant