US boats used their DGs all the time vs smaller targets. Typical hits for gewrman guns to sink targets were about the same 60-80 hits based on the uboat.net narratives.
Here's a jap merchant ship that we have in game as she actually was in 1942:
4 3 inch or better DGs, and as many AA MGs (either 25mm type 96s or 13.2mm HMGs). Her sisters were similarly armed. This is
Nagara in mid 1942, she was like this before the war started.