Had an interesting moment testing out the HE rounds with the Tank Destroyer.
Spawn in on the US side on Armored Shield, grab a M1128 Stryker and start driving to C to capture it and have our team spawn with the Gunship. A Havoc pilot tries to circle-strafe me, comes too low, I fire, boom, no more Havoc.
Closer to the end of the round, I'm in another Stryker trying to give support to some infantry trying to capture (I think) D. Havoc pilot comes back, tries to circle-strafe, I fire and kill both the pilot and gunner, but the Havoc is only disabled! And it lands right behind my tank, so my squad-mates get out, repair the chopper and fly away. I look at my HUD and see I forgot to switch back to the regular AP rounds. I was in utter disbelief!
In short, compared to regular cannon rounds, the HE rounds have a larger splash area and is better at killing infantry, but are not as effective against vehicles.