I think the most recent rules are from the updated Dred Mob, available as a free PDF on the Forge World website.
http://www.forgeworld.co.uk/Downloads/Product/PDF/d/Dred_Mob.pdf
As far as it's
40k-ness goes, it's a regular Heavy Support unit for the Dred Mob and as legal as any Forge World unit is oin your gaming group. As far as I know, it is not available to 'regular' Orks and must be allied in.
As a final note, it's extremely mediocre because the whole thing is reliant on hitting with a single BS2 shot, and will hit itself half as often as it hits the enemy.