Use is primarily shooting prone and using a chunk of wood as a rest. Rifles typically have long barrels (>42"), more than 1" across the flats, double set triggers, and open sights.
Pretty amateur stuff from Mark, but you can get a feel for the rifle and shooting. Note that the rifle has a wood block secured to the barrel to provide a consistent resting point for the barrel. Moving the rest position can change the point of impact enough to widen the group. Often the barrel rest will have a level to ensure the rifle is always level when shot.
To start off, a sighting target is shot. Three shots are used to establish a group. That sighting target is pinned over the X so the group will impact at the crossing of the X. One shot per X.
The Target is an X. Originally scratched on a flat section of board in a black smudge of soot. Target distance is 60 yards. Scoring is the measure from the center of the ball to the crossing of the X. Target strings in a 10 shot match have been less than 5 inches.
Youll see anything on the chunk line from traditional flint and percussion sidelock guns just like you'd see anywhere to underhammer monsters. Like previous post said.....most will be over 42" and heavy. 48" range is a common length. 50-58 caliber pretty common but you'll see others too. Personally i shoot a 48"x1 1/4" traditional flintlock and a few underhammers up to 60" long and 1 3/4" with octagon or round barrels, and anywhere from .475 to .54. caliber.
Look up "Bill Large Old Hanner".....one of the early log shooters and probably still the largest fullstock rifle ive seen. Although a fella did have a custom fullstock built 10-15 years ago that he brought to a couple of matches that was 8'+ but more of a novelty than competition rifle. Stopped by his house for a barrel and he literslly had to lean it in the corner due to his low ceilings