From Aero Precision:
 
The PRO Upper Receiver Group built around our PRO 5.56 Cold Hammer Forged Barrel that is paired with our PRO 5.56 BCG. Included ATLAS R-ONE Handguard, BREACH Charging Handle, and VG6 Delta muzzle device round this assembly out into a premium turnkey solution for your next build.
 
NOTICE: The muzzle device on this upper receiver is not pinned or welded. All NFA rules and regulations apply.
 
Includes:
 
13.7" PRO 5.56 Cold Hammer Forged Barrel
PRO 5.56 Bolt Carrier Group
M4E1 Threaded Upper Receiver
ATLAS R-ONE Handguard
VG6 Delta Flash Hider
Mid-Length Gas System
About Cold Hammer Forging
 
Cold hammer forging differs from other common barrel manufacturing processes. Rather than drilling a blank and using cutters, buttons, and reamers to machine the chamber and rifling into the barrel, a cold hammer forged barrel is made by forming the barrel around a mandrel that has the rifling inclusions precision ground into it. This process is referred to as "cold" hammer forging because the process is done at room temperature and the metal is not heated.