In the source material, the HE aspect is still meaningless (just a lore aspect) and while there's a chance it might pierce armor, it is slim and it's more of a lore thing as it would pierce to a point and inject the HE into the mech, allowing shells to produce some damage. (In fact the largest caliber AC/5s in Battletech actually only produce 1.66667 damage per shot and require 3 of them to make 5 damage. Then again ACs in general are more in tune with IFV autocannons and less in tune with MBT main cannons; those would be Mech Rifles which MWO doesn't have.)
If the Autocannon ammo was not HEAP, then they would be grossly inferior to Mech Rifles (and Mech Rifles are considered obsolete military tech and is more often considered non-military tech because unless the target is soft aka a Barrier Armor Rating of 7 or less , the Mech Rifle automatically loses 3 damage against armor. Yes this means the Light Rifle does 0 damage. All mechs and modern military tanks have a BAR of 10 and all other Mech Weapons have an anti-BAR of 10 meaning they are evenly matched).
This is what an autocannon would look like in
real life. Add explosives to impact.
IFV using an autocannon. These autocannons are of a caliber used commonly by AC/2s and very specifically the Blackjack. Ironically, it would take about 10 shots of the Puma's AC to produce "2" damage which is what the Blackjack's 30mm AC/2 does in lore, which is summarized as a single 'use' in a 10 second timeslice by tabletop (which with advanced rules AC/2s can deliver 2 uses at a high risk of jam and explosion) MWO gives us a "use" or rating per shot instead.
A Mech Rifle. (Specifically, a Light Mech Rifle, which does zero damage to mechs.)
Autocannons cap out (in lore) at about 2,000 meters. Mech Rifles cap between 3,000 and 4,000 meters.
ACs (the AC/2s which fire the least shots or have the least recoil of all ACs) are accurate up to 690 meters (without undue difficulty of an average pilot), while their highest accurate range in a pilot with the Sniper skill is 1,380 meters. This of course is the range where you'd have to land all your shots just to do 2 damage..
The heaviest
Mech Rifle has an accurate range (for an average pilot with undue difficulty) of 540 meters, and for a pilot with the sniper skill, their highest rnge (with the same difficulty for the sniper skillset pilot) is 1080 meters to deliver a single shot of 9 damage (which cuts to 6 against mechs and modern main battle tanks). Of course, a Mech Rifle could only be fired up to twice with a significant cut to accuracy on the second shot and risk to the weapon in 10 seconds... An Autocannon can net 2 ratings for the same risk, but when your equal comparison of an AC/5 can net 5 damage instead of 6 for the same ranges and yet can still be used as a machine gun and with a single 'rating' you can even choose to spread your ammo against two targets... it isn't any wonder the Mech Rifle became obsolete in lore.
After all, an 8 ton Mech Rifle carries 6 shots in a ton of ammo and serves as a single magazine. An 8 ton AC/5 can get the same range, carry 20 magazines (called Cassettes0 of X shots each within the same 1 ton of ammo. Sure it might take the whole cassette to do that 5 damage, but there's 20 uses in a ton (BT not MWO, MWO gives you more) versus 6 uses of what amounts to 6 damage.
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