That would have all the same problems a full-sized Nerf AEG would have, but be pistol-sized. You need a *massive* battery, motor, and gear drive train *in addition to* all the gear slugs already got going on. A CO2 or LPA/HPA rig is more feasible, and a minimized stryfe is an easier route to compact semi auto (working on a 3d printable design for just that, actually).
Just out of curiosity, are there any homemade Nerf AEGs?