The joke could work better if they weren't taking a diameter's cannon (or is it caliber's gun?) as reference for cannon longitude.

Wait... Is this Rance 6 reference?

The scence when he was captuerd and was told to do something interesting so he undress in front of the demon lady to show her his hyper weapon?

rom_collector said:

The joke could work better if they weren't taking a diameter's cannon (or is it caliber's gun?) as reference for cannon longitude.

No, I think she's saying something more like "I'm pretty sure that's barely a 5.1cm L/1 cannon" (i.e. the barrel length is 1x the calibre)

Nope, it's just slang for saying that his 'main cannon' is small, crappy, goes off at a moment's notice, and can't lower its shields.