FBI HRT Geissele Mk4 Federal Rail and Barrel Kit - 11.5" Colt LPGB

The FBI HRT starter kit — a Geissele Mk4 Federal (OD Green) 10" rail paired with a Colt 11.5" factory government-profile barrel and Surefire SOCOM flash hider.

• Geissele Mk4 Federal 10" OD Green rail: current-production MLOK rail in the FBI-issue OD Green, with laser-etched T-markings
• Colt factory 11.5" government-profile barrel: CMV 4150 chrome-lined, 1:7 twist, 5.56 NATO, with Colt-integrated low-profile gas block (pinned)
• Choice of Surefire SOCOM flash hider: 3-prong, Warcomp 3-prong, or the previously-restricted military 4-prong — all DLC-coated, all SOCOM RC2 compatible
• Complete kit, not pre-assembled: Geissele barrel nut, bolts, Allen wrench, gas tube roll pin, Surefire indexed washers, and a Rocksett tube — everything the build needs
• Built in partnership with Potomac Armory to hit the right balance of parts without the flagship price

The FBI Hostage Rescue Team adopted the Geissele Mk4 Federal rail in OD Green as part of the bureau's current carbine configuration, and the combination of the OD Green Federal rail with a Colt 11.5" government-profile barrel has become the reference baseline for a clone-correct FBI HRT build. Charlie's partnered with Potomac Armory to assemble this kit — the rail and the barrel are the two parts that define the build, and getting them right makes the rest of the rifle straightforward.

The rail is the current-production Geissele Mk4 Federal in 10" OD Green, with laser-etched T-markings for top-rail indexing. The barrel is a factory Colt 11.5" government-profile, CMV 4150 steel with a mil-spec black phosphate finish, chrome-lined, 1:7 twist, chambered in 5.56 NATO. Colt ships this barrel on its EPR performance series rifles. The low-profile gas block is a Colt factory unit — a shaved-down remnant of the A-frame front sight base, integrated to the barrel and held by two taper pins through the barrel (removable when needed).

The gas tube is a Colt premium military-contract carbine-length stainless tube. The muzzle device is your pick of three Surefire SOCOM flash hiders, all of which index to any Surefire SOCOM suppressor (including the SOCOM RC2). The basic 3-prong is the most common issue hider. The Warcomp 3-prong is a compensating variant — effective, but the blast is diverted up and to the right, which becomes very loud on an 11.5" barrel for anyone shooting next to you. The 4-prong was the military-issue hider that sat outside the civilian supply chain for years.

The kit is engineered to fit mil-spec forged upper receivers. Potomac recommends forged uppers from Colt, LMT, Daniel Defense, Bravo Company, FN, or Rock River — billet receivers sometimes time poorly to the Geissele Mk4 rail.

What's in the box

• Geissele Mk4 Federal 10" OD Green rail with laser-etched T-markings
• Colt 11.5" factory government-profile barrel, CMV 4150, chrome-lined, 1:7 twist, 5.56 NATO, with Colt-integrated low-profile gas block (two taper pins, removable)
• Surefire SOCOM flash hider — your choice of 3-prong, Warcomp 3-prong, or 4-prong, all DLC-coated
• Colt premium military-contract carbine-length stainless gas tube
• Geissele barrel nut and mounting hardware
• Small Allen wrench for Mk4 rail tab torque
• Gas block roll pin for the gas tube
• Surefire indexed washers for flash hider timing
• Rocksett tube for muzzle device retention

Note: the kit arrives in a box, unassembled. Build photos show a previous-generation Surefire flash hider for illustration.

For the same kit with a Daniel Defense barrel in place of the Colt, see the HRT Federal Rail and Barrel kit with DD barrel.

We've got your six on the clone-correct details — the Colt barrel markings, the Geissele T-marks, and the Surefire index are all the pieces the community checks first. Order yours today and we'll get the kit shipped.