Wulf is correct, a chunk gun is fired in the prone possition with the barrel resting on a "chunk" of wood... There is no weight limit to the rifle, and shaders are normally used to cover the sights and cut down the glare off the barrel and stabilize the sight picture...
The target is an "X"... A dead center hit is called a "spider"... Normally, (though the NMLRA does shoot 3 shots at the X), you shoot one shot per target at the "X", and it is a string measurement. The target may be on paper or a black charred shingle with an "X" scratched on it...
Matches can consist of any number of targets, and shots, (one shot per target), fired from the chunk at 60 yards.
Your rifle does not have to be zeroed in dead center because your allowed to use a "spotter" pinned on your target as an aiming point so the ball will strike center... This is why the sightin period is important, and you want as good of group as possible becauce you are not allowed to adjust your sights when the match starts. You can adjust your spotter pinned to your target however...
You might try;
www.nmlra.org, or search for "Chunk Gun Matches"... If you find something that says "Bill Large Match" go to it, or Alvin York Match, Pall Mall, TN go to that as well...
Good luck in your search!