Question:  The Ordnance QF Hotchkiss 6 pounder gun were introduced to defend against watercraft capable of independent operation underwater usually referred to as "boats" rather than what?
Sentence (from "QF 6-pounder Hotchkiss"): There were many variants produced, often under license which ranged in length from 40 to 58 calibers, but 40 caliber was the most common version.
Relevant evidence: No