HK MR556A1 Accuracy Test

In this range test the Ransom Rifle Master Tester Mega 2 is used to test the accuracy and repeatability of the HK MR556A1. Hornady 75gr ammo is used during testing.

HK MR556A1:

  • Caliber: 5.56 x 45mm
  • Stock Extended Length: 37.68 in
  • Stock Retracted Length: 33.90 in
  • Width: 3.07 in
  • Height: 9.45 in
  • Barrel Length: 16.5 in
  • Weight w/o Magazine: 8.6 lb
  • Magazine Capacity: 10/20/30 rounds
  • Trigger Pull: 7.64 lb
  • Barrel Profile: 6 lands & grooves, right twist, 1 in 7 in
  • Sight Radius: 14.60 in
  • Sights: Various mechanical or optical sights
HK MR556A1 Features
HK MR556A1 Features

HK MR556A1 Features:

  1. Compensator – Bird cage style flash hider is screwed to the end of the barrel and helps compensate for muzzle rise during firing.
  2. Gas Block – Directs the expended gas from the gas port hole in the barrel and pushes the piston rearward, pushing the piston rod and bolt carrier rearward (defined in more detail in the cycle of operation). Contains an attachment point for the optional HK folding front sight.
  3. Modular Rail System (MRS) – The HK MRS is a free-floating detachable rail system handguard that allows the shooter to place individual rail components and accessories anywhere along the length of the seven free sides of the forearm using the slotted interface. Self capturing rail segments can be mounted in any position along the 9 inch forearm/handguard (a 14 inch MRS is also available and is standard on the Competition Model). Two integrated Quick Detachable (QD) sling attachment points are present just forward of the receiver on each side of the MRS. Optional, positionable quick detachable sling mounts can also be attached to slotted interface. The MRS has an octagonal, ergonomic profile and provides a comfortable gripping area for the shooter’s support hand. The MRS attaches to the barrel extension nut with a standard type SW11 screw. The MRS is present on most MR556A1 models produced after 2014.
  4. Upper Receiver – The upper receiver is a flat-top design with an integral MIL-STD-1913 (Picatinny) rail that provides support for the barrel, FFRS, gas system, and bolt carrier assembly. It has a case deflector and ejection port. It connects to the barrel and barrel socket. The piston rod is guided through the upper receiver by a bushing and activates the bolt carrier. The charging handle is a part of the upper receiver as is the forward assist and ejection port cover. The upper receiver is made from an aluminum forging.
  5. Charging Handle – Allows the operator to chamber or clear a round and cock the firearm. It also provides for initial charging of the firearm. The charging handle is locked in the forward position during firing to prevent injury to the operator. An enhanced charging handle with an extended catch is available on the MR556A1.
  6. Selector/Safety Lever – Allows the operator to select the mode of fire and place the firearm on safe. The selector lever is manually activated and unlike competing AR systems, can be switched to safe even if the hammer is not cocked.
  7. Trigger – When activated by the operator, initiates the firing sequence. Located inside the lower receiver, the trigger when squeezed with the firearm loaded and off “safe” will fire the gun, “SEMI-AUTOMATIC” will fire one round each time the trigger is pulled. The MR556A1 uses a proprietary HK two-stage trigger for optimal accuracy.
  8. Trigger Guard – The trigger guard protects the trigger. It can be lowered by depressing the detent with a pointed implement, allowing the use of heavy gloves or mittens to be used to fire the rifle.
  9. Bolt Catch/Bolt Release – The bolt catch/release is manually operated. When the bolt carrier is pulled fully to the rear, the bottom of the bolt catch/release is pressed by the operator and then the bolt will be captured. To release the bolt, simply press the top of the bolt catch/release. The bolt catch/release is also automatically activated when firing the last round in a magazine.
  10. Sliding Buttstock – Attached to the firearm via the receiver extension. The sliding buttstock is ambidextrous and adjustable to 6 positions for length of pull by pressing in on the rear portion of the Release Lever. It also contains the rear sling attachment points. Many optional buttstocks can be attached to the MR556A1 as long as they fit on the Receiver Extension (Buffer Tube), which has MIL-SPEC cross sectional dimensions.
  11. Forward Assist – Ensures that the bolt is fully closed and locked. The forward assist is used when a round is not fully chambered. You can push on the forward assist until the bolt seats the cartridge fully into the chamber. It will return to its rearward position by spring pressure. It can also be used to silently load the firearm.
  12. Case Deflector – Built into the upper receiver, deflects the cartridge cases away from the operator.
  13. Ejection Port Cover Assembly – The ejection port cover is attached to the upper receiver and is spring loaded. It will open automatically if the charging handle is pulled to the rear or if the firearm is fired and can be manually closed at any time. It closes over the ejection port to prevent sand, dust, etc. from entering the chamber. It should remain closed when the firearm is not being fired.
  14. Barrel Assembly – The chamber accepts the cartridge for firing and directs the projectiles down range. The barrel is cold hammer forged and has standard land and groove rifling with a 178 mm (1 in 7 inch) twist. The barrel is pressed into the upper receiver assembly and held in place by the threaded barrel nut.
  15. Locking Screw – The Modular Rail System is secured using a standard SW11 screw while the Free Floating Rail System is fastened with a locking screw that can be tightened and loosened by using a 5 mm Allen wrench. Both lock into the forward extending barrel locking bushing. Fastening and positioning of the locking screws on the bushing ensures maintenance of zeroing on any sight system which has previously been attached.
  16. Pivot Pin – The pivot pin allows the upper receiver to pivot when the rear takedown pin is depressed. When both the pivot pin and rear takedown pin are depressed, the upper and lower receiver groups can be separated. The pivot pin is captive and should not be removed. Both the pivot pin (front takedown) and rear takedown pin have locking shafts that need to be pressed while the pins are depressed and engaged.
  17. Magazine – Holds cartridges ready for feeding and provides a guide for positioning cartridges for stripping and chambering. The magazine provides reload capabilities for sustained firing. The HK 30-round steel magazine and optional 20-round steel magazine have a steel follower that will not cant during the feeding process. The spring is stronger and more reliable than standard magazine springs. The MR556A1 can be used with other high quality magazines of all capacities.
  18. Magazine Catch/Magazine Release – Holds the magazine (in place) in the magazine well and allows the operator to release the magazine and remove it from the firearm. The magazine catch is activated automatically when the magazine is properly inserted into the magazine well. The magazine catch is spring loaded and when the button is depressed, the magazine will drop free from the firearm when this control is activated.
  19. Lower Receiver – Contains the fire control system for the firearm. The lower receiver contains the hammer, trigger, disconnector, sear, magazine well, magazine catch/release, bolt catch/release, selector/safety lever, buttstock assembly with buffer and recoil spring, pistol grip and trigger guard. The lower receiver is made from an aluminum forging and provides a beveled magazine well to ease reloading.
  20. Pistol Grip – The pistol grip is made so it can be used by a left or right handed shooter. It has a trapdoor on the bottom so extra batteries, small tools, or other accessories may be stored. Other AR-type pistol grips can also be installed.
  21. Rear Takedown Pin – When the rear takedown pin is depressed, it allows the upper receiver to pivot on the front pivot pin. This pin is captive and should not be removed. Like the pivot pin (front takedown pin), the rear takedown pin has a locking shaft that needs to be pressed while the pins are depressed and engaged for disassembly and reassembly.
  22. Buttstock Release Lever – The buttstock release lever locks the telescopic buttstock to the tubular rear buttstock extension. The buttstock is adjustable in length in 6 positions.
  23. Rear Sight – attaches to the MIL-STD-1913 top rail of the upper receiver.
  24. Front Sight – attaches to the MIL-STD-1913 top rail of the MRS or the FFRS handguards.
  25. Rear (Diopter) Sight – proprietary HK sight used on many HK firearms, attaches to the MIL-STD-1913 top rail of the upper receiver.
  26. Front (Flip up) Sight – proprietary HK sight attaches to the gas block, folds down when not in use.
  27. Free Floating Rail System (FFRS) – Present on most MR556A1 models produced 2011- The FFRS is a free-floating detachable rail system with four quadant MIL-STD-1913 (Picatinny) rails that can mount a variety of accessories including sights, scopes, grips, lights, laser aimers, bipods, and sling hardware. The bore sight alignment of aiming devices is repeatable (no re-zeroing required) when the Free Floating Rail System is removed and reinstalled on the firearm. The FFRS is removed and reinstalled with a 5 mm Allen wrench.