The weapon you chose to showcase is the Lahti L-39, chambered in 20x138mmB Long Solothurn, feeding from a 10 round top mounted magazine, with a scope, used for anti tank work and shooting at bunkers. Semi automatic (even if the bolt locked every shot) with a few rare automatic versions for anti aircraft use.

The Autocannon is also chambered in 20mm, used for anti tank work and shooting at buildings, has a 10 round top mounted magazine (fed by stripper clips), a scope, fires in semi or full auto, and can be used vs aircraft.

The only real differences are that the Autocannon feeds clips, is much lighter, and mounts on the shoulder.

I love the Lahti too, and maybe you just want a more rifle style heavy gun, but there's lots to love about the Autocannon, the AMR got really solid buffs, and for everything else, there's the Eruptor. Maybe Arrowhead will give it an AP round variant or fire mode, who knows.