P90-like half-length darts magazine mechanism flywheeler.
Designer’s selling the files for $25
https://www.etsy.com/listing/785112876/bulwark-nerf-blaster-full-auto-bullpupDimensions:
> The largest part is the thumbhole stock it approximately 8 1/2" long. The front grip section is a tad longer than 7" as well. I use a CR-10 mini for my printing.Will apparently also fit in a Prusa mini
Rough estimate of hardware and material cost by another user,
> it's probably about a spool and a half of filament, just as a guess. That'll run you around $30. Add $15 for motors, $10 for wheels, add in $15 for the pusher mech parts, assume you already have a LiPo, and add $5 for screws, wire, odds and ends, and you've got $75 