The Braverman Stinger Pen Gun chambered in 22LR. These come up very rarely for sale and are definitely a unique piece of history.  Braverman designed and manufactured this one and avoided the NFR rules by making it fold into a "gun shape."  You would carry it retracted in it's "pen like" state where it just appeared to be a stubby pen. In a couple of seconds you could deploy the Stinger pen gun and fire it's single shot fairly rapidly.

Buy it online here - https://shop.jerarms.com/product/bj-braverman-pen-gun-rare-22lr-with-sheath?preview=MTE2NDoyMDIxLTA4LTE5