That pellet upper is a cool concept but they're not even breaking 600 fps with it. I've seen
pistols that do better, and my $120 break-action single shot rifle does 1000 fps.
I like that Matech BUIS...will have to check one out.
Here's my homeland security device. Upper is a POF 415.
Here's my ultralight carry-all-day ranch hand (14.5" barrel makes for easier handling too).
This is my buffalo harvester--.458 SOCOM.
The latter two lowers are from Cavalry Arms. Ugly as sin but lightweight and comfortable, and come with the best warranty on the planet.