[quote=@WilsonTurner]
[@Foster]
I dunno. Imagine a Bradley with a mortar instead of a Bushmaster.
[/quote]

The AMOS still has ammo-racks eating space in the troop-compartment, that was actually a problem with the Bradley itself due to the bulk of the TOW missile package easily reducing the troop capacity officially to 6, and could further reduce the load down to little as 2 dismounts in the recon-variant (M3 Bradley).

This problem is universal to all mobile-gun or fire-support conversions of personnel-carriers, it either significantly reduces its ability to carry passengers inside a combat-zone, or has to do-away with it altogether in order to function at all.
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-An automated grenade launcher would be able to saturate the area quite easily.

*It would also have issues being a problem for a FRAME's armor

This said, mounting a light mortar on the engine-deck of an armored vehicle (in addition to lashing-on ammo-crates) is a thing.

I was assuming it was a battery of frame-mounted mortars acting cooperatively.