I installed a 16" Rainier Arms Select SS barrel on my Colt 6920. It's a mid length, 416 SS barrel chambered in 556 with 3R rifling. I'm extremely pleased with it. Excellent accuracy and easy to clean. I'd buy another in a heartbeat.
Specs on barrel.
Features:
Barrel Length: 16"
Gas Block Seat: .750
Rifling: Polygonal 3 Groove
Weight: 31 oz
Stainless Steel: 416R
Feed Ramps: M4
Gas System: Mid-length
Chamber: 5.56MM
Finish: Matte Stainless
Twist: 1�8
Thread Pitch: 1/2�28
