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

$735.55 - $799.95
ships from our shop in 3 to 5 days (+/-)

FBI HRT Federal Rail and Barrel Kit - 11.5" Geissele - Colt heavy (FBI) barrel

Starter build kit with Geissele Mk4 Federal FBI rail, includes gas block and gas tube

Charlies Custom Clones brings you the HRT Federal Green rail and barrel kit.  This is a great start for an FBI HRT clone rifle build.  Now being offered with the HRT heavy 11.5" barrel.  Charlie's has partnered with Potomac Armory to bring you a balance of the right parts.  This is the current production Geissele 10" OD Green Federal (FBI) rail with laser etched T-marks. 

We feature a Colt factory 11.5" Chrome-line FBI heavy barrel, in 4150 chrome-moly-vanadium steel (CMV) with a Troy gas block.  The barrel originally came with a front sight post, and that has been replaced with the low profile gas block.  You might notice a small groove where the forward pin was placed for the FSB.  We show you pics of the groove, which has been chemically treated. The gas tube is factory Colt as well.  So, you get the best in performance with a factory Colt barrel, complete with factory Colt markings.  With this offering, you have your choice of a three-prong AAC flash hider or the Surefire 3-prong SOCOM flash hider.  The AAC flash hider is a high performance flash hider, but does not support a suppressor.  The Surefire supports the SOCOM suppressor line - most all current production 5.56 and 7.62 suppressors.  For those living in a state that does not allow flash hiders, you should choose the Surefire and ask in the comments to have a brake installed instead.

The rail and barrel kit will fit a mil-spec upper receiver.  Potomac recommends forged receivers, like Colt, LMT, Daniel Defense, Bravo Company, FN or Rock River to name a few.  Billet receivers sometimes do not do as well timing to the Geissele rail.

Note:  All items arrive in a box, unassembled.


  • Geissele Mk4 Federal 10" OD Green rail with laser etched T-markings
  • Colt 11.5" factory SOCOM / FBI heavy profile barrel, CMV 4150 steel with mil spec phosphate black/gray finish, 1:7 twist 556 NATO chambered barrel with chrome lining
  • Colt CAGE code markings on barrel
  • Your choice of the AAC Blackout Flash Hider, part number 64177, or the Surefire SOCOM 3-prong 5.56 flash hider with DLC coating
  • Troy premium low profile gas block with two set pins
  • Colt premium military contract carbine length stainless steel gas tube
  • Geissele barrel nut and bolts to attach
  • smaller Allen wrench for tightening Mk4 rail tabs
  • gas block roll pin for attaching the gas tube
  • Surefire indexed washers for attaching and timing the Surefire flash hider, or a crush washer for the AAC Blackout flash hider
  • Small Rocksett tube to apply to barrel to attach the flash hider

Note:  for the same kit with a Daniel Defense barrel, see here:  HRT FBI barrel and OD Green Geissele Federal Rail Kit with DD barrel