Squad Designated Marksman Rifles

The AMU’s Designated Marksman Rifle is yet another example of competition shooting’s positive influence. The Custom Firearm Shop Service Rifle gunsmiths have built and continue to service over 375 Squad Designated Marksman rifles based on their experience of building and maintaining over 200 M16 uppers and lowers for competition use. Service Rifle Teams firing M16A2/A4 rifles with free float tubes have dominated National and Interservice championships and broke all records held by the M14/M1A.

What started as an experiment to create AR-15s capable of winning at Camp Perry has found its way into the hands of designated marksmen in Iraq and Afghanistan. The AMU uses these rifles to train Designated Marksmen at Fort Benning and abroad. The 101st and 82nd has fielded them in combat with exemplary results. Here’s how they are put together.

The DMR is a standard M16A4 fitted with a stainless steel matchquality barrel, clamp-on front sight A-Frame, free-floating M1913-railed hand guard, bipod with quick release adapter and two-stage trigger. All the modifi cations can be applied or reversed in about 30 minutes by direct support 45Bs with proper training. The total current cost for these modifi cations is $1,200. Issue M16A4 barrels are turned to a small diameter under the hand guards to reduce weight and for M203 mounting.

This small diameter allows the barrel to whip during rapid-fi re engagements causing shot dispersion. Additionally the front sling The modifi cations are made to standard M16A4 rifles and can be applied in thirty minutes by trained direct support personnel (MOS 45B trained by the AMU CFS.) The total cost for these additions is $1,200. swivel is attached to the barrel via the front sight base and any tension on the sling during firing causes changes of impact.

The match replacement barrels are 0.840" diameter profile, twenty-inch long stainless steel barrels. The portion of the barrel that is visible is dimensionally the same as an issue barrel. The muzzle is threaded 0.5"/28-TPI to accept standard attachments. The replacement barrel is assembled with a clamp-on front sight A-Frame and proof fi red using a M197 Proof Cartridge as prescribed for all M16 barrels.

Further, all DMR upper receivers are function fi red for reliability and accuracy using A059 Ball and AA53 ammunition. The issue hand guards are replaced with a float tube, removing all external pressures from the barrel. The replacement hand guard manufactured by Daniel Defense is made of aircraft grade SAE 6061T6 aluminum and has external M1913 rails on all four sides, like issue RAS hand guards, but will not begin to rotate or twist after field use.

The issue barrel is removed and anew barrel is inserted into the upper receiver with the barrel nut and hand guard nut behind it next to the receiver, tightened 30 foot-pounds using a combination tool and torque wrench and re-torqued a total of three times. On the fi nal tightening, alignment of the gas tube hole is checked in the barrel nut with gas tube hole in the upper receiver. If it is aligned no further tightening is required.

If the gas tube hole in the barrel nut is not aligned with the hole in the upper receiver

Notable examples for just the M16 developed by the CFS include improved A2 rear sights, free float hand guards/rails, and magazine-length 77 grain projectiles (Mk 262). the barrel nut is tightened to align the holes, not to exceed 80 foot-pounds. Two aligning pins are inserted into the holes in the barrel nut at 3 o’clock and 9 o’clock as viewed from the muzzle. The hand guard threaded end is slid over the barrel and onto the alignment pins.

The hand guard nut is slowly engaged onto the hand guard while gently wiggling the front up and down to facilitate tightening the nut. Once the hand guard nut is hand tight, it is is clamped in a padded vise and snugged up with a spanner wrench. The gas tube is installed into the clamp-on front sight base using anew spring pin. The front sight base and assembled gas tube is slid onto the barrel, rotating the assembly to align the gas tube with the hole in the barrel nut.

There should be a 0.025-0.030inch gap between the front sight base and the shoulder on the barrel when it is properly fitted. Once in the proper position the hex screws are tightened to 30 inch-pounds while rotating the base to align the front sight as near horizontal as possible to eliminate large windage adjustments when zeroing the rifle. The compensator with the concave compression washer is installed with the small diameter towards the barrel.

The center slot of the compensator should come up to 10 o’clock when tightened by hand and then tightened to 12 o’clock with a combination tool and torque wrench. Installing the compensator from further away from 12 o’clock will result in bore constriction which is detrimental to accuracy. The M16A2/A4 trigger system is replaced with a two-stage Geissele Automatics match trigger. The issue safety/ selector is retained and functions as issued, but the burst capability has been eliminated.

A Harris HBLMS bipod is fitted with a La Rue throw lever mount. AG