Some Articles, First from Soldier Systems on the Son Tay Raid GAU-5/5A and how it compares to the modern M4A1 CQBR
Next we have a four part series from Jeff Gurwitch (9 combat deployments, 26 year Career) published between July 4th, 2012 and December 30th, 2018 for Defense Review detailing the evolution of his and his teammate's rifles as well as the "Competition to Combat Crossover".
Colt T-12 Pen Gun, very little is known about the gun, not even the date of manufacture, fires a comically tiny .12 cartridge
Quiet Special Purpose Revolver (QSPR) of the Vietnam War, utilized captive piston ammunition (for a Soviet Equivalent silent revolver see the Ots-3

for reduced firing noise which was said to be about 110db. Made from cut down and modified S&W Model 29's (In Bizzaro World instead of packing a roaring handcannon, Dirty Harry wielded a mouse fart snub nose) Fired Tungsten birdshot out of a smoothbore barrel. The guns were intended for Tunnel Rats, Somewhere between 11 to 250 were made, one was apparently lost and at some later time was alleged to have been used in an attempted murder in California (Whenever you read anything regarding this gun this will come up, But I am yet to have seen anything actually backing this claim up)
Before there was the Colt 9mm SMG, Maxwell Atchisson (same guy who would make the AA-12) was experimenting with the concept.
Top 2 are open bolts, bottom is a closed bolt and intergally suppressed, using a modified .45 MAC suppressor, modified to be wipeless. Something about the middle gun for me.

Atchisson also made, this, the C.R.A.C.K or "Counter Action Recoiling Compensated Kick" which is certainly an aesthetically jarring design. M1 Garand receiver, BAR trigger group and parts from a Thomspon, 7.62x51mm chambering. By judging by the cutout in the stock and the forward spring, I think the receiver was supposed to reciprocate rearwards slightly while cycling.
He also had a penchant for .22's seeing as how he made the original AR conversion kit and two unique rifles (one of which is somewhat reminiscent of a Hi-point carbine
The Packer
This one has a barrel which is "slightly curved"
