I like Zenark's idea. The whole airship carrier full of airships is rather amusing, but that type of thing has been done before. I don't think smaller airships would be carried, but given that there are monoplanes existing in the Guns of Icarus world, I could see something like the USS Akron (a rigid airship that carried Sparrowhawk pursuit biplanes) or heck, Don Karnage's Iron Vulture from TaleSpin (am I dating myself here?). That would be rather neat to see monoplanes being launched from underneath a behemoth airship.
And putting a heavy gun in an airborne craft is nothing new. Aircraft that I can think of are the British Mosquito Mk.18, the Ju-88P, and the B-25G Mitchell. All with super-heavies (for aircraft). I think something like 75mm or so.
But that brings to point what HamsterIV was talking about. The airspeed velocity of a coconut-laden swallow. No, not really, but it is a matter of weight ratio. Airships, steampunk or otherwise, will always be carrying the lighter-weight weapons. They need to. Whereas ground installations can have as heavy and as powerful as they possibly can field; weapons that can not only out-distance the effective range of an airborne arsenal, but out explodey it too. Hitting a stationary target is fine, it sucks when that stationary target can also bring to bear more firepower and withstand more beating than you for a longer period of time.
The hitting of moving targets isn't much an issue, because all you need to do is hit the section of the airspace that they're in. One thing my great-uncle, who was a waist gunner for a B-24 during the Second World War, always mentioned, was how terrifying the flak fields were. Because it was so thick, so precise, so impersonal, and so invisible. All you saw was the puff of black smoke, not the fragments of shrapnel nor the rounds ascending. And there were tens of thousands of rounds all trying to occupy the same airspace that you were in.
Also, in the concept that you can just bombard a defense into submission and it'll work, well, I have an island to sell you on that idea. It's called Iwo Jima, and the American brass thought that after a three days of bombardment (some of which was 14-inch munitions) that the Japanese would be so shell-shocked that the American forces could just relatively waltz in. Instead, the Japanese kept their heads down, dusted themselves off, and some 36 days later, after 26,000 American casualties and almost 22,000 Japanese dead, Americans had secured the island.
Defense and offense are always vying against each other, and ground to air defense is always a better bet than air to ground assault (given relatively equal technology advances).
