Avenger Strike Fighter

The Bastard offspring of the Warthog and the Stuka.

The Avenger Strike Fighter is an Imperial Navy close-range ground-attack aircraft meant for destroying armor and other heavy targets. It does this with the Avenger Bolt Cannon, the S6 AP-2 D2 Heavy 10 gun around which the flyer was built, as well as 2 lascannons and 2 hardpoints that can mount anti-infantry or anti-vehicle weapons. Since it was supposedly used a lot by Sebastian Thor's forces during the Age of Apostasy, the Sisters of Battle see it as an instrument of divine vengeance (through superior firepower), so they use it a lot. The Avenger model is produced by Forge World, released to coincide with Imperial Armour Aeronautica (the book containing the rules for it and other flyers, what with Warhammer 40,000 6th edition having flyer rules).

Forge World was very obviously inspired by the real-world A-10 Thunderbolt II (or "Warthog"), a ground-attack aircraft built around the tremendously powerful GAU-8/A Avenger Gatling cannon. It shoots 30mm depleted uranium shells so fast that it doesn't go "Dakka," it goes BRRRRRRT. Despite this shared characteristic, the Avenger's overall shape is more akin to a Ju-87 Stuka: both have inverted gull wings but the ones on the Stuka are more triangular. It is a ground-attack plane after all. Interesting analogy by the author here. I'm sure it's purely coincidence, but a point of trivia worth noting is that renowned Stuka ace, and therefore the grandfather of close air support, Hans Ulrich Rudel (who is also the most decorated German soldier of World War 2, the only one to receive the highest German award to non-generals -Iron Cross with Golden Oak Leaves, Swords and Diamonds- , and destroyed a battleship, the Soviet Marat, hundreds of tanks and vehicles and had 9 enemy plane kills, making him an ace) actually consulted for the USAF during the conception of the A-10. Cold War realpolitik made for some real funny bedfellows.

Like the modern A-10's cannon, the Avenger's Gatling cannon isn't all that great against battle tanks and similar vehicles because 40k game mechanics don't use side armour to resolve hits from flyers like they do hits from barrage weapons (unless you are a chad and play 30k, where you can penetrate from the side or the rear). Just like modern tanks, almost all 40k tanks (except siege tanks, like the Vindicator) would be side armour (AV10-12) on the top, and therefore quite vulnerable to the hail of S6 shots. That said, with the Strafing Run and Tank Hunter rules (the latter available only in the Horus Heresy version), its Gatling cannon is actually quite effective against AV 10 or 11 targets. Taking antiarmor hard points, though, allows you some tactical flexibility as you can use the big gun for mowing infantry while the hard points take out armor.

In 8th edition with the removal of AV, its namesake main gun struggles against most vehicles for a different reason. Unlike its big cousin bolt cannons the Castigator and Vulcan, with which it shares the same Strength and AP, its bolts do 1 damage each rather than 2. This seems to shift its use away from being an anti-vehicle weapon that can target infantry (like autocannons) and toward being an anti-infantry weapon that can target vehicles (like assault cannons). Its relation to the assault cannon is kind of like that of the heavy bolter to the big shoota: similar weapon, but firing bolts instead of bullets for improved penetration, though the Avenger has better range and fire rate. But if we focus on what the main gun can do rather than what it should do, it does have impressive utility. T7, 8 and 12+ are bad targets for it, but with it's AP -2 pretty much everything else is fair game. Ideally, you'd want to point it at dangerous armoured infantry, and on a flying platform that often means strafing their devastator/dark reaper equivalents. It's also good for bypassing necron quantum shielding, and decent at shaving the last wound needed to kill or cripple a vehicle that just ate the Avenger's lascannon fire. Barring that, you just point it at whatever few of your other units are able to take out. That can mean T9/10 models that lascannons will struggle against, light vehicles that more specialized weapons don't want to bother with, infantry in an inconvenient place, and more besides.