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 found an alternative use for this gun, to sneakily assassinate business rivals. His first assassination using the Shrink Gun was senior executive Frank. After Cross perfected his version of Pym Particles, he did not use the Shrink Gun again; probably because he thought of it as a failed experiment or because he found it too dangerous or impractical for some reason.