I do not think you need to be very picky on your choice of model guns. If it is field artillery your are looking at, only 6 out of the 13 batteries had the new 1863 4 lb rifles guns. Here are some images of these:
The rest had smoothbore calibres from 6-84 lb. Your choice of model also depends on what scale/ratio you intend to use on the tabletop. The 6 rifled batteries had each 8 rifled 4lb guns while the other 7 smoothbore batteries had one of the following configurations:
6 x 6lb "kuglekanon" - firing solid cannonballs (or canister)
2 x 12lb "granatkanon" - a howitzer firing shell.
6 x 12lb "kuglekanon" - firing solid cannonballs (or canister)
2 x 24lb "granatkanon" - a howitzer firing shell.
6 x 24lb or 36lb "kuglekanon" - firing solid cannonballs (or canister)
2 x 84lb "granatkanon" - a howitzer firing shell.
An image of a smoothbore 12lb gun:
I will personally use some ACW guns