The Upper Receiver Group - Improved (URG-I), also styled URGI, is an upgraded upper receiver assembly for the M4A1 carbine adopted by United States Army Special Operations Command (USASOC). First fielded in 2018, it replaces the upper receiver, barrel and handguard of the SOPMOD Block II M4A1 with lighter, longer-lived components while retaining the existing M4A1 lower receiver. Because only the upper assembly is changed, rifles fitted with the URG-I retain the M4A1 designation.
The kit's principal features are a Daniel Defense cold hammer-forged barrel that returns to the lighter "government" profile, a mid-length gas system, and a lightweight Geissele Automatics free-floating handguard using the M-LOK mounting standard. The package was optimized for the U.S. military's M855A1 Enhanced Performance Round.
Development The URG-I originated as a USASOC effort to upgrade the M4A1 carbines used by its subordinate units, which were already equipped with M4A1s using the SOPMOD Block II accessory package. Geissele Automatics and Daniel Defense collaborated on the upper receiver groups, which were initially ordered for the 75th Ranger Regiment and the U.S. Army Special Forces groups. Geissele subsequently marketed a civilian "near-clone" of the assembly, which differs from the issued item chiefly in that its muzzle device is pinned and welded to meet the minimum barrel length required of non-NFA rifles.
Although informally referred to by some as "Block III", because the modification affects only the upper receiver, URG-I–equipped rifles remain Block II weapons.
Design Barrel and gas system The URG-I uses a cold hammer-forged, chrome-lined barrel with a 1:7 rifling twist, supplied by Daniel Defense. In contrast to the heavier SOCOM profile barrel found on Block II rifles, the URG-I returns to the lighter "government" profile. The standard configuration pairs this barrel with a mid-length gas system in place of the carbine-length system used on the M4A1 which lowers bolt carrier velocity and reduces felt recoil and parts wear, increasing reliability and extending the service life of the weapon. The barrel is fitted with a SureFire four-prong flash hider that doubles as a mount for SureFire's SOCOM-series sound suppressors.
Handguard and furniture The defining external feature of the URG-I is the Geissele Super Modular Rail (SMR) Mk16, a slim free-floating handguard that incorporates the M-LOK accessory-mounting standard in place of the quad-rail Picatinny handguard of earlier configurations. The assembly also includes a Geissele ambidextrous charging handle and a low-profile gas block. For the contract-issued configuration the upper receiver and bolt carrier group are built to military specification by Colt.
Variants
- Standard-issue configuration, with a mid-length gas system and a Mk16 rail - A shorter civilian configuration, with a mid-length gas system and a Mk16 rail. Not in military service - A close-quarters configuration with a Mk16 rail
Users
United States Army Special Operations Command 75th Ranger Regiment U.S. Army Special Forces Air Force Special Operations Command
Successor In May 2026, Naval Surface Warfare Center Crane released a solicitation for the Hypervelocity Improved Capability Assault Rifle (HICAR), an effort to replace or upgrade the URG-I with a system able to fire both standard 5.56×45mm NATO and an emerging high-pressure "hypervelocity" cartridge, with the stated goal of extending effective range from 300 to 600 meters.
See also Close Quarters Battle Receiver Mk 12 Special Purpose Rifle
References