In following up on the subject of firing rates some more, I noted the following in the passing of Fort Morgan in Mobile bay: according to Bergeron's "Confederate Mobile" the passing took 45 minutes during which "fourteen heavy and several lighter guns" fired 491 shot and shell. Assuming ~20 guns in all fired continuously then time between firing was = (20 guns * 45 mins)/491 rounds = 1.83 mins per round per gun. The rates of the more active guns were probably considerably better than that since some of the pieces probably either lacked range or position to fire continuously.

Of course during infantry assault, the firing rate would be expected to be considerably greater because the duration of the firing would be short, and the need pressing. Overheating or other concerns would be far less important. Canister would likely be rapidly loaded and fired compared to normal siege or long range artillery duel rates (with canister aiming would be largely unnecessary other than in a very general sense.)