M4 Blackwater SIR Upper Receiver Group, Mod 1 with #50 M-CV RAS rail

$1,409.00 - $1,679.00
built to order, allow 4 to 6 weeks +/- to ship (including time for pin/wedl

M4 SIR Upper Receiver Group, Mod 1 with #50 M-CV RAS rail -14.5" pin/weld

Early GWOT upper used by Blackwater, SEALS and, US Army Special Forces, USMC Recon teams

The M4 Blackwater SIR upper receiver is a unique piece of history at a time when every manufacturer was trying to answer how to adapt an M4A1 carbine and shorter barreled rifles to quad rail platforms that both provided Picatinny rail space for weapons accessories and also provide a free float rail to not weigh down the barrel with all this extra weight.  A.R.M.S. entered the scene with the SIR rail, which came in a number of flavors. 

The most popular was the #50 bi-rail system, that incorporated a top "Swan rail" over the receiver and connected to the SIR / RAS rail.  The number of units using SIR rails was very small and mostly in the very early years of the global war on terror (2001-2003), before other rails from Daniel Defense and Knights Armament became more ubiquitous.  The early rails we are calling Mod 0, and the Mod 1 is the ARMS model #50 M-CV which is the rail alone without the top section.  Also, the M4 SIR was paired with Colt and FN barrels ranging from 10.3" to 14.5"

To be fair, there are limited sightings, and we know that Blackwater contractors used the SIR rail extensively, so we are taking some liberties, but wanted to bring you something unique and fun to shoot and quality product. 

Charlie's offers the M4 SIR URG in two models, both of which function the same and have close to the same level of quality, but serve different preferences and budgets.  The basic model has a military grade forged upper receiver and milspec phosphated / chrome lined BCG and charging handle, and we also offer an upgraded model with Colt receiver, BCG and charging handle.  From a function standpoint, the only real difference is that Colt features a dry lubed interior to their receivers.  Otherwise, the quality is close to the same, but having Colt gives the shooter perhaps a more realistic look and feel to the history of the entire M4 SIR rifle / upper receiver.


  • Colt M4 profile 14.5-Inch 1/7 twist 5.56 NATO Mil-Spec chrome lined MP tested barrel, chrome lined and phosphated exterior with Colt installed front sight post
  • M4 style carbine gas DI system
  • A.R.M.S. #50 M-CV S.I.R. free floating rail manufactured from high-grade aluminum and anodized with Type-3 hard-coated in dark gray, weighing less than 9 oz.
  • M4 milspec receiver, 7075 aluminum with Type 3 military grade anodizing, white T-marks on the top rail
  • milspec charging handle
  • milspec bolt and bolt carrier group with C-158 or 9310 MP tested bolt and tool steel 8620 carrier with chrome interior and phosphate exterior
  • choice to upgrade to Colt receiver, BCG and charging handle
  • Surefire SOCOM 4-prong military grade flash hider, pinned and welded to the barrel to achieve 16.1" overall barrel length.

photos by Charlies Custom Clones, ARMS and Sgt Ronald Schumer