Looks like a very minor issue, however automatically setting the engagement mode to all targets would probably be benefitial.
Not sure how well it turns out with other shell switch abilities where the target is not as clearly defined as here. The most important thing is probably to keep it consistent.
No to making it a single shot ability though. That would make it somewhat similar to the ZiS barrage in terms of functionality.
Regarding the cannister shot in general, I think there could be some range regained with vet, so that spamming them is not oppressive since they need to stay in range of small arms themselves at vet. At vet 1-2, when medium tanks arrive and the usability of the M42 diminishes both regarding tanks as well as against better infantry squads and upcoming elites, it could maybe get a small range buff of +5-10 to reinforce the AI role and make it less vulnerable.
I thought this whole board's warcry was "consistency!". Target tables are a no go because interaction between squads becomes inconsistent, faction asymmetry is ok but only when everyone has consistent access to the same tools, and so on.
Tell me how consistent it would be for a shell switch changing the target priority on your selected unit? Sure, the M42 won't be doing any damage against tanks with its AI rounds, but neither does the Sherman, and it keeps its prioritization settings anyways.
And if you do make those consistent between each other, then the ISU is the odd man out because its AI shells CAN do damage to tanks (at least as far as I remember, unless I'm mixing it up with the KV-2).
Hell, though, if I am wrong about the ISU then screw it, disable vehicle prioritization while in AI shells mode for all of them. I don't really see the downside.