Had a look around the inside of an AS90at a army cadets thing when i was younger.
The breach on it was huge.
1m x 0.7 x 0.7 (ish) and came all the way back into the firing area, and back into the forward position in quarter of a second.
(they showed us a vid of it)
it was rather scary, how little space there was in there, and how quick it moved, and how heavy it is.
bit like but a proper gun 😉 (there is an internal camera shot half way through the vid)