The Sicaran Battle Tank is out and looking awesome!
The weapon is a Herakles Accelerator Autocannon, which sounds like something that will be an AA weapon with more punch than a standard autocannon. Rules won't be out till the next Horus Heresy book. It costs £72 or about $ 112 US.
From Forge World:
The primary armament of the main Sicaran variant is the sophisticated Herakles pattern accelerator autocannon, a superior rapid-firing and highly accurate weapon. It fires shells at far higher velocities than a standard autocannon, enabling it to successfully track and engage swift moving targets and pinpoint vulnerabilities in enemy armour with lethal precision.
The Sicaran Battle Tank, designed by Phil Stutcinskas, is a complete resin kit. This model is available to pre-order now and will be despatched from Friday 30th August.The rules for this model can be found in a forthcoming Horus Heresy Book.