Bush and Hornblower refer to the shell which Hornblower extinguished as a "five inch". The cannon was clearly no more than a 12 pound gun which would have had a bore diameter of about 3.7 inches. A five inch gun would have been too large to be easily moved by hand as shown in the scene.
After the long boat is destroyed the French are shown loading
a shell into a field piece. It makes a rolling sound as it goes down the barrel. This would be inaccurate. A shell fit tightly enough to require ramming, and a shell would have been carefully positioned in the barrel so the fuse would face away from the powder charge.