Hand Cannon: You can, indeed, fit some bloody powerful mass drivers into handgun-sized weapons, these days – although even with recoil compensation, etc., it helps to have some of those military-grade musculoskeletal reinforcements – and some way to brace yourself – if you plan on firing the things comfortably or with any reasonable degree of accuracy.
There are even a couple of handgun-sized slugguns on the market, if you feel like a one-handed gyroc-grenade launcher is just what the doctor ordered. (And Eye-in-the-Flame Arms, outré as ever, sells a sluggun derringer – which, yes, you could theoretically load with an slugfire antimatter grenade – although even by the Empire’s loose standards, the crossover market between “small, concealable suitable-for-waistcoat-pockets-and-ladies’-purses gun” and “can snipe buildings” is… not exactly huge.)
[As a side note, while it is entirely in keeping with the Eye-in-the-Flame design process – which is to say, getting as high as possible on creativity-enhancing nootropic drugs and ignoring entirely the coquetries of practicality – I was a mite concerned about the reader-credibility of this particular example of their products.
Then I learned about this real-world product, a derringer chambered for .81/20 mm, which is to say the type of shells used in Vulcan autocannon. And that they’re planning a 30 mm version.
I rest my case.]