The Shrink Gun is a prototype tech piece that works with Darren Cross's replication of Pym Particles. Although it was originally meant to shrink living beings, it instead kills them by condensing their tissue into a tiny puddle of protein substance.
Darren Cross created to the Shrink Gun in order to shrink living beings. While testing the gun instead of just shrinking the subjects it killed them. Cross used this alternative use for this gun to sneakily assassinate business rivals. His first assassination using the Shrink Gun was senior executive Frank.