Not a "build" per so, but over the summer I picked up an SA Saint AR pistol in 5.56. Wasn't crazy about the blast and loss of velocity, so then swapped in a BCM upper in 300 BLK. Great pistol round. Designed for the shorter barrels unlike the 5.56. If you load, you can load subsonic. If you want to pay $200 for a stamp, you can also add a suppressor and be almost silent.