Mike's Hobby Home has this primer on American Civil War artillery. The information includes a good table on the various pieces, including data on the shot size, range of the weapons, elevation and the number in use.
John Martin has written a set of free wargames rules for the American Civil War based on Games Workshop's Warmaster. I think that things have come full circle here, because Warmaster obviously took much of its inspiration from the seminal ACW rules set, "Fire and Fury." Not that GW would ever admit that. After all, according to the Evil Empire, miniature wargaming is the "Games Workshop Hobby", which precludes the acknowledgement of any other rules set or miniatures.
The Jackson Gamers' have published the "Continuous Fire Fight Rules for the American Civil War. They write:
Robert Whitfield and Larry Brom developed these in 1984 after we refought the battle of Fredericksburg in 25mm scale, and the Confederates were unable to stop the Yankees by firepower. In these rules there is a �deadly ground� in front of an Infantry regiment (and to a lesser degree, in front of an artillery battery). Any enemy unit in that ground WILL be fired at until it is destroyed, or falls back voluntarily or involuntarily, or forces the units that are firing on it to withdraw.
The mechanisms of play such as Infantry and Artillery fire, Movement, Close combat (Melee), and Morale are taken straight from Larry Brom�s rules with little if any change. The unique facet of these rules is that once a unit moves into the �deadly ground� it will be fired at and will test morale, it may then return fire and the original firing unit will test morale. Then both units settle into a routine of firing and morale testing, till one fails morale or voluntarily falls back (or is destroyed). This is a difficult concept to grasp, because as miniature wargamers, we are so used to a turn sequence of: �we move, we fire, we test morale, we fight melees, we have another turn�.
Fire in this rules set is deadly. If one player persists in holding an exposed position, his unit or units will be destroyed by fire in one turn. Players must grasp the concept (so dear to the hearts of infantrymen) of bugging out if the fire-fight begins to go against them.
Bob Bergman has written a set of free wargames rules for the American Civil War called Seeing The Elephant. Designed for two different regimental and a brigade scale, the rules use a card activation mechanism, combined with a Morale Check. Firing is done by adding Fire Factors and rolling a die; Melee by figuring odds and rolling a die. In all, a comprehensive, well-thought out set of rules.