If you're going to use quad 50's at a field up an M16.
IMO, depending on field size, sml, med, & lrg is what type and amount of ground guns would need to be in place.
Small - two 20mm fixed gun mounts, two single 40mm bofors mounts.
Medium - three 20mm fixed mounts, one single 40mm bofors mount. one twin 40mm bofors mount
Large - four 20mm mounts. two single 40mm bofors mounts. two twin 40mm bofors mounts. one quad 40mm bofors mount
20mm Mount
Single 40mm Bofors
Twin 40mm Bofors
Quad 40mm Bofors, Oh yeah! 