This is a Midwest Industries known as the Upper Receiver Rod, model MI-URR.
The Midwest Industries upper receiver rod is the ultimate tool for barrel nut installation or removal and is excellent for removing stubborn/seized barrel nuts without damaging upper. This receiver rod is also designed for muzzle device installation and will not damage the index pin. This receiver rod can also be installed horizontally or vertically in a bench vise and works with any standard M4/M16 barrel extension. Please note that this is NOT COMPATIBLE with 9mm and rimfire barrels and is also NOT COMPATIBLE with DPMS G2 308.
Can be used to install forward assist/port door cover
Compatible with forged and billet upper receivers
Machined from 4140 Ordnance grade steel
Durable Black Oxide finished for years of use
Receiver mounted sights can be left installed