Hydra Flak Tank
The Hydra Flak Tank is a specialised weapons system used by the Imperial Guard, designed to take out enemy air support.
Overview
The Hydra is based on the Chimera chassis, replacing its troop-carrying abilities with a big gun turret on top, munitions and targeting systems for its weapon, and the fuel to haul it all around. Its weapon is normally associated with fixed platforms, but the Hydra is made for the purpose of deploying anti-air support in non-permanent locations and as a mobile backup.
The Hydra's primary weapon are four long-barreled Hydra Autocannons, weapons that are basically just Autocannons with extended barrels. This does give them better range and penetration (it has the exact same penetration as the standard Autocannon), though, as it gives more time (barrel length) for the projectile to accelerate (begins decelerating after leaving the barrel). They are mounted on the side of the turret, allowing a cone of fire that encompasses pretty much anything located above the Hydra. Though the individual weapons are not very strong (strength 7, wounding most other vehicles on a 4+ or 5+), the sheer number of shots they pump out (8 shots per turn) means that they will often take the enemy down with sheer volume of fire. Though not having the most dakka in the Imperial Guard, the Hydra certainly is up there.
Now with 8th edition out in the open Flyers simply get a -1 to hit against all models without the FLY key word. The Hydra Flak Cannon gets a +1 to hit models with the FLY keyword, and so ignores this, but hits everything else on a 5. However, there are lots, and lots and lots of models with the FLY keyword. Anything with a jump pack (Assault Marines, T'au Crisis suits, etc) have the keyword, as well as skimmers, jetbikes and of course flyers. What this means is that the old days of just mulching anything is nearly, nearly back. Further with the changes to the AP system and twin-linked, this thing now is a MEQ's worse nightmare. That said it costs 93 pts now, but given the points increase to most vehicles this isn't even that expensive.
Now, for those of us with curiosity, there is some fluff with the Hydra being a triple-A gun platform. That is Anti-Air-Artillery, the stuff in World War Two that made the black clouds of shrapnel. So, if the Hydra can use explosive ammunition, why can't the infantry Autocannons use the same? Possibly because flak rounds are unsuitable for use by heavy weapon teams due to increased recoil and greater danger of danger close accidents... Or the simple fact that heavy weapon teams just can't get their guns pointing high enough on their tripods to make AA fire feasible. Unless you’re a soldier in WW2, apparently, since they didn’t have any such problem. No, this is about killing infantry with black clouds of death from man-portable autocannons. And in all honesty, it appears that Hydra's fluff schizophrenically jumps between Bullet Hell and black clouds of Shrapnel Doom™. I know the infantry wouldn't use their Autocannons to shoot aircraft; they'd use the cannons like they already do. You don't need to put shrapnel into it, just use the space for that for more explodium.
It's important to remember that we're talking about the Adeptus Mechanicus here. It's entirely possible for them to have an STC fragment for Autocannon flak shells, but until someone can convince them that they found an STC for other types of Autocannon shells in the Manufactorium's crapper, anyone who suggests deviating from the design of the flak shell is going to get lobotomized for tech-heresy.