>(I'm not sure if this is a variant of the
>M113 that actually exists. There are a million-zillion variants of
>the M113, so it's entirely possible.)

The question is not so much "is there a M113 variant with an autocannon," as "which one?"

The Danes only recently retired the M113A2 Mk I DK.
Egypt's EIFV mounts a M2 Bradley turret.
Lebanon mounts a ZU-23 twin autocannon on some of theirs.
etc.