400 x 20mm rounds (wikipedia says the scout carries 400) would take up 5.2 cubic feet if they were in 4 M548 containers. The feeding mechanisms would give you a little bit more.
It helps but it is still a very small vehicle.
Edit. Revised calculations for linked ammo. (originally had done unlinked/bulk)
Sources
http://olive-drab.com/od_firearms_am..._m548_40mm.php
http://en.wikipedia.org/wiki/Cadilla...Commando_Scout