|Canister was close in meant to put up what could probably be called a virtual wall of lead to take down anyone approaching a field piece. I'd probably equate it more to a CIWS than a WMD. The Civil War Artillery page (http://www.civilwarartillery.com/) gives the follow descriptions in it's glossary
CANISTER: A metal cylinder made of tin, iron, or lead, with a removable thin iron top. A heavy iron plate is usually located between the canister balls and the wooden sabot at the bottom. The cylinder contains iron or lead balls which are arranged in rows with sawdust packed between them. The top edge of the vertical cylinder wall is bent over the iron top plate to help keep the canister contents in place and the bottom edge is nailed to the wooden sabot. Canister was designed to be used close range against enemy troops with the desired effect being that of a huge shotgun blast. It was recommended for use at ranges under 350 yards. A 12-pounder canister for the Napoleon smoothbore contained twenty-seven iron balls while the 12-pounder howitzer contained forty-eight iron balls. Canister was used in field, mountain, siege, and seacoast weapons.
CANISTER-SHOT: A canister-shot is a metallic cylinder about one caliber in length, filled with balls and closed at both ends with wooden or metal disks. They are supplied for all guns. For 8-inch canister, and all those of less caliber, the envelope is made of tin, while canister for the larger calibers have an envelope of iron. The bottom of 15-inch canister is made of two thicknesses of 1-inch hard wood, crossing each other, and put together with wrought-iron nails clinched. A spindle, with a wrought-iron handle passing through the center of the canister, is riveted on the bottom through a square plate. All other canister have bottom-heads of one thickness of hard wood. Top-heads are all made of white pine. The case is notched, turned over the heads, and tacked down. The balls for all canister are 1.3 inch diameter, and the number used varies with the caliber. To give more solidity to the mass, and prevent the balls from crowding upon each other when the piece is fired, the interstices are closely packed with sawdust. See Case-shot, Projectiles, Rifle-canister, and Siege and Garrison Ammunition.