[Seraphim38]

It is called a KAK Shockwave brace, which is designed to be used as a forearm brace.

After purchasing it, I found that it is easier to use it as a cheek rest because it gives you a much better head position with the red dots.

For those who don't know, any rifle that is less than 26" overall length has to be classified as a pistol, or else it must be registered with the ATF. That process takes about a year and is no less rigorous than the process for registering a suppressor. The KAK Shockwave brace is part of a group of pistol braces used to rest against your forearm or cheek, and can be mounted without having to register your pistol as an SBR or AOW . Putting it up to your shoulder, while it can be done, is not what it was designed for, and isn't very effective anyway.

Yes, the HK is chambered in .22LR. It is a fantastic "plinker" or "trainer", and is my 7 year old son's favorite gun.
The other AR is a 9mm Palmetto State lower/Karri's Guns upper build with an 8" barrell. It takes Glock 33 round magazines.
Pistols are a Canik TP9SFx 9mm pistol with a Vortex Venom red dot and TLR-1 HL. Also pictured is a CZ SP-01 9mm.