I outright stopped using HE-P and Canister because I just can't kill infantry in a TRE with them, unless they are in trenches because amusingly, HE-P pushes them out of their foxholes. I've only been using HE-P when against a BTR-60 or BMP's flank and I want to save a HEAT round.
TRE?
HE round can be effective against infantry but you have to either get the round to impact at an angle in front of them. Or at least that's what I'd assume works best, does the game model different shrapnel blast patterns? Against trenches, if you're at a great enough distance, try increasing your superelevation so your rounds will drop in at a higher angle. Regardless though since the M393 doesn't have a timed fuze (IRL) I don't really think it's intended for APer use since not having the ability to airburst over or drop detonate into a trench really hinders your options. That, together with the M494 not seemingly working means the in-game M-60's APer capacity is really hindered. Disappointing considering that Steel Fury let you adjust timed fuzes and the shrapnel shells were quite effective.
Also the M-60s smoke rounds, being White Phosphorous, would have some deadly effect on infantry, as well as the morale impact. I have to see if this is modeled, haven't tried it before.