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Guide to HK Locking Pieces


Identification of HK
 Locking Pieces
An HKPRO Special



Note from HKPRO-- The following article was published in the January, 2001 issue of Small Arms Review Magazine, and is reproduced here in its entirety, with permission from the author, for the obvious relevance to students of HK small arms. Text and photos by Dan Shea

One of the “mysteries” of HK ownership is the locking pieces.  After numerous reader calls, letters and emails in regard to what is the “right” locking piece, we put this together this SAR Photo ID guide.  We are only covering the locking pieces that are German HK manufacture, and only the ones that are pertinent to the US market.  There are somewhere in the vicinity of 300 different locking pieces that HK in Germany has issued at various times, to address various situations.  We do not cover any of the aftermarket locking pieces either.  SAR would like to thank Jim Schatz of HK for his assistance in putting this information together.  This guide should cover the HK locking pieces you are apt to run into in the U.S.

L-R: HK91/G3/SR9 (no marking-45°), G3K marked 18 (50°), PSG1 marked 19 (40°), MSG90 marked 23T-T is for Titanium (40°) and MSG90 30T for different ammo pressure. (40°)
The locking piece that we are referring to in this article is the HK locking piece.  It is found inside the bolt carrier, and the bolt head mounts on to it then locks into place.  The firing pin slides inside the locking piece as it goes into the bolt carrier.  This interlocking metal “puzzle” is a smooth working machine, and the two round rollers in the bolt head, and their relationship to the angles on the forward slopes of the locking piece and those within the barrel extension are the key to the whole HK system.

In order to safely fire a rifle, the rifle must be stronger than the expanding propellant gases that are seeking to escape; preferably by pushing the projectile out of the barrel. If the projectile stopped moving, then the weakest link in the chain of construction of the firearm would give.  This is always a negative experience.  From the beginning of firearms design, the gun builder has had to factor this simple and seemingly obvious fact into his design.  In a single shot firearm this is fairly easy to accomplish.

G41 marked 9A (40°) and G41K marked 22A (45°)

Enter the bolt actions, then the automatics.  Bolt lug failure is uncommon in most bolt-action designs, as the locking lugs are robust, and lock up to the solid metal of the receiver.  The operator pulls the trigger, the primer ignites the propellant, the propellant deflagrates and the gases expand, and push the bullet out of the bore.  Operator unlocks the system to insert another cartridge.  Pretty simple, really.  In the early part of the last century, fully automatic firearms were being designed, and the semi automatics came into being as well.  The designers realized early on that there were numerous ways to harness the recoil or byproducts of the firing process, in order to cycle an action, reloading a firearm.  Timing of the unlocking then became an issue.

Without getting too far off the subject at hand, we need to look at an occurrence that set the designers “free.”  Free from the bulky receivers and manufacturing processes they had been saddled with.  This was the advent of locking the bolt face to the barrel, through a barrel extension.  There are numerous variations on the theme, including trunion locking, but the basic idea is that you only need to have the strength to contain the ignition process until the peak of initial firing has passed.  Stoner used the cam unlocking system on his rotary bolt head with its multiple interlocking lugs into the barrel extension, and was free to use aluminum receivers.  Kalashnikov used a somewhat standard dual locking lug that was also cammed, allowing him to eventually go to sheet metal on his receivers.  Timing is a critical part of the uncamming action here.

HK’s design is rooted in the late World War II German prototype, the MP45.  This radical design used moveable rollers to lock the action shut, then to release the bolt into recoil after the peak of pressure had passed.  This design followed into the Spanish CETME for reason obvious to those familiar with HK history, and on to the G3 rifle and its offspring, the MP5 and HK33 series.

L-R:  HK33/93 marked 3 (58°), HK33/93 marked 7 (sometimes used in HK53--55°), HK33 marked 8 (Malaysia--53°),  HK33 marked 11 (Sometimes early HK23E and HK13--Unk angle), and HK53 marked 15 (65°)

Basically the bolt group comes forward, stripping a round off the magazine and following it into the chamber.  As the round is seated, the bolt head “bottoms out” on the cartridge base and chamber mouth.  Tolerances here are critical of course, but for simplicity’s sake we will continue.  The bolt carrier still has more distance to travel, under spring tension from the return spring.  Since the bolt head is forward to the extent of its travel, and the locking piece is attached to the bolt carrier, the locking piece is driven forward inside the bolt head.

HK11E marked 9 can be used in HK21E depending on ammo pressure (40°), HK21E marked 17 can also use 9 for diff pressure (36°) and HK23E marked 20 (70°)
The two locking rollers are at rest inside the bolt head, with their outer edges in line with the bolt head surface.  As the locking piece comes forward, the wedge shaped section of the locking piece presses against the rollers and pushes them outwards.  The locking rollers move into matching recesses in the trunion that the barrel is mounted to.  When they reach full extension, the bolt head is locked forward.  (There is much more involved in this process, and the tolerances of the parts, but we are simply looking at the locking pieces here.)  In the rifles, the locking lever found on the bolt carrier snaps into the bolt head providing additional locking strength for the more powerful rifle cartridge.  Locking levers are not present on pistol caliber HK roller locked weapons.  The firing pin can now travel forward if the hammer strikes it, and primer ignition can occur.

As the gases expand, tremendous pressure is exerted in all directions inside the cartridge case.  Since the projectile is the “moveable” part of the system, it travels down the bore.  Mr. Newton explained that every action has an equal and opposite reaction, and this is paramount to our discussion at this point.  The bolt face receives an equal pressure, and exerts it rearward against the locking rollers. There are pressure peaks and spikes that are readily measurable.  Until the projectile leaves the bore, the internal pressures are hazardous to the operator--meaning that the locking rollers must remain in a locked position, or the bolt and bolt carrier assembly will be coming rearward with more energy than the system is designed for.

MP5/HK94 and MP5K/SP89 locking pieces L-R:  Standard MP5 /HK94 not marked (100°).  Early MP5SD marked 5 (120°), MP5SD marked MP5 Action (115°), MP5K marked 16 (110°), MP5K-PDW for use with suppressor and stock marked 80° on earlier pieces and 28 on later ones.

The locking rollers are meant to stay locked during the peak of pressure, then to gradually move into the bolt head and allow the bolt carrier system to go into recoil.  The angles on the forward “wedge” of the locking piece dictate the timing of this event.  These are keyed to certain cartridge pressures and certain configurations of firearms.  All cartridges are not created equal, and any variation to the system as a whole may affect the timing.

HK hunting rifle locking pieces. SAR was unable to locate the factory angles on these.  L-R:  HK940 marked 30-06, HK770/SL7 marked with caliber (.308), HK630, SL6 marked with caliber (.223) and HK770 in .243 marked with caliber.

Thus you have two different locking pieces for 10mm MP5s—a high impulse and low impulse.  Adding a stock and suppressor to an MP5K will necessitate a different angle, and thus a different locking piece.  MP5SDs need a different locking piece than an MP5, and so on.

MP5/10 and MP5/40 locking pieces L-R:  10mm low impulse marked LO 24 (90°), 10mm high impulse and recently the .40 high impulse marked HI 25 (60°), .40 cal marked 26 (80°)

Interchanging locking pieces can affect bolt gap, which can adversely affect reliability, rate of fire and durability.  It is therefore not recommended. 

1HK11 Alt40
2HK21 Alt50
4HK21 Alt45
6HK13 Alt58
10HK13 Alt55
12HK21A1 Alt45
13HK53 Alt70
14HK13A3 LMG58
27MP5K Alt90
29MP5KA1, N70

Herewith is the SAR guide to HK locking pieces.  We hope this resolves a lot of questions in this regard.  Above all else, compare markings and the angles on the locking piece forward wedge to help attain a positive identification.  Don’t use parts that you are unsure of.  The angle to be measured for identification is in parentheses in the captions.  See the diagram supplied by HK to show the measurement angle.  There are many other possible locking pieces that you may encounter, that might have been factory installed, not just a field mistake.  To our knowledge, this is a pretty comprehensive look at the ones that HK imported to the United States.  As always, SAR would be happy to get any other information from the readers in this regard, we will file it, then do an addendum as more information surfaces.