Hydra Flak Tank
The Hydra Flak Tank is a specialised weapons system used by the Imperial Guard, designed to take out enemy air support.
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 (they require a 6 to glance armor 13), the sheer number of shots they pump out (4 twin-linked shots per turn) means that they will often take the enemy down with sheer volume of fire. Thanks to their superior targeting systems, enemy vehicles and jetbikes don't get cover saves from going flat out or turbo-boosting, so they will eat jetbikes for breakfast. Though not having the most dakka in the Imperial Guard, the Hydra certainly is up there.
If needed, the Hydra can commit its weapons to anti-infantry purposes, ripping through anything wearing carapace armor with ease and forcing even MEQs to rethink their chances. At the cost of 70 a piece and being able to field up to 3 in the same Heavy Support slot, you may want to rethink how much dakka you want for your points. Unfortunately, the almighty Hydra has now been rendered unable to hit ground units (except skimmers - take that Xeno scum!) with its amazing Autocannons (even with twin-linked) with any sort of accuracy, making it close to useless against ground targets. A good tip if you are going to be firing at ground targets, move, you're already forced to snapfire that means you can move at cruising speed and suffer no negative penalties to shooting.
Before you pile 6 Hydras in anticipation to 7th edition flyer spam, note that Hydra is only effective against light flyers and skimmers since STR 7 Autocannons are as effective against gunships (Heldrake I'm looking at you @_-) as S&M outfit is against horde of daemonettes. Also the lack of interceptor means that said gunships can barrage hydras with whatever missiles they decided to pack themselves with, potentially (likely) turning your hydras into slag/wreck/flying debris in turn 2, which we admit is a reason why you might want to pile 6 hydras into a list to make sure some survive but you have better options.
Really, if you seriously want anti-air, get vendetta squadrons since flyers can choose between Skyfire or no Skyfire at start of each turn. And 3 twin-linked Lascannons beat 2 twin-linked Autocannons any day. Although with 6th edition codex, Vendettas are more expensive and carry less models (6). Oh and you get 2 and a half Hydras for 1 Vendetta. Quite a bummer eh?
Not anymore (kinda). 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 suites 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 105 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. 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.
What he's saying is, that since AAA ammunition for the Autocannon clearly exists, why can't there be alternate ammunition for the Autocannon in the tabletop game? An explosive filler for Autocannon ammunition could be rather interesting, you're probably only looking at S3, AP6, small blast, heavy 2. Though that would ultimately detract from the Missile Launcher.
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.