First,is the tube lined with a seamless steel liner?
Second,is the breech radiused where the liner & the endplug meet inside.
Iron guns need to be lined before being fired live.

J. Paul Barnett of South Bend Replica's has quite a collection of iron tubes of various pedigree that have been sectioned in half. Once you see those, you become a firm believer in the "Poured solid, bored & lined" method of cannon manufacturing.

Saboted & steel straped balls do not appear to be more accurate than loose loaded balls, and in fact seem to be slightly less accurate.Perhaps due to the inconsistent way that the ball seperates from the sabot & strapping.

Smoothbore cannon are frustratingly innacurate from shot to shot. Rifled guns were a vast improvement.