Basic cannon take 50 seconds for the oldest tier, 45 seconds for the 12 pounder and 6 pound horse to reload. Accuracy is extremely poor with any obstructions in line of sight. Accuracy improves by the time the targets are within 60% of range.
Smaller cannon tend to be more point accurate, even with basic round shot. 'Point' fire is the ability to track and accurately engage a target of man size.
Larger cannon not so much. Larger the gun, the slower the reload and worse the 'point' accuracy. Larger guns and howitzers are traditionally only 'Area' fire accurate. Immobile targets, walls, buildings, entrenchments.
Try doing a 'Grand Battery' (
http://en.wikipedia.org/wiki/Grand_Battery) style approach; take at least 3 sets of cannons to your next battle. 5 is best. Place 3 cannon directly ahead of your main body. Place two other cannons to a flank side (left or right of your main body, 300 yards or so) and place a picket group of infantry or horse outside the flanked cannons.
Reserve your fire; toggle it off as soon as the cannons unlimber. Allow the enemy to advance to 75% of your cannon range and fire two sets of the guns from the main battery. Let them shell the advancing enemy. Wait for the enemy to close ranks, prepare cannister shot in the flanking guns and the third gun. Fire the third battery in the main group. That load of canister grape will give the enemy pause, possibly enough for the first and second guns to reload.
When the enemy is 200 yards in front of your main battery and body of troops, fire the flank guns with cannister. Place all guns on hold (pause the game, select each unit and hit the stop order) and charge your infantry forward, ahead of the guns. Enable their fire order and watch them devastate the enemy. Enfilade and oblique attacks are your friends. Always cover your main body with enfilade fire. If you have no place to do so safely, look for rises or dips in the terrain to create a defiladed kill zone or fighting position.
Anything left standing is then hit by cavalry from your flank. I drew a few diagrams up last night, then failed to save them. I am making a monster guide, but generally anything by Clausewitz applies.