I posted links to something like that in the PDW thread a couple months ago. Basically, a manually cycled star wars blaster. There's a big thread on .300BLK forum. Buy your own side charger upper, drill and tap the bolt carrier for a handle. Either D&T the port to plug it with a set screw or shrink a sleeve over it. Cut the back off the bolt carrier, and you can run a light spring and PIC-mount, folder BRN-180 style. It's quiet.
On the other hand, you can also get a CVA Scout rifle or pistol that's equally quiet. Doesn't eat from AR mags, but will be a lot cheaper and less fiddle-frackery.