Having worked with the KP-15 for the WWSD2020 project, I got a hankering to know how a tiny SBR carbine would work using a KP-15 lower. There is no option for using a pistol brace, of course so this was built as a registered short-barreled rifle. I used a Faxon 10.5 inch 5.56mm pencil-weight barrel, a Tacstar (?) carbon fiber handguard, a standard KE upper, and a complete WWSD 2021 lower. The hand stop is from Slate Black Industries, and the optic is a Holosun 503CU.
Why not use .300 Blackout? Basically, because 5.56mm is a lot more convenient and I don’t need any of the benefits of .300.
Why use a fixed stock? Weight, strength, and simplicity. I wanted the light weight of the monolithic polymer stock, and it is a stronger design than a collapsing type.
I’m really happy with how the experiment turned out! The WWSD Commando has an overall length of 29.5 inches, and weighs (with the optic) 5lb 2oz (2.3kg). It has a bit more snap and bark than a proper-length barrel, but is not unpleasant to shoot by any means.