Text Size

Internal Operation

Internal Operation and
Roller Locking System


Integral to properly understanding the majority of HK firearms is a knowledge of the roller locking system.  Most all HK firearms use the roller locking system as the method of operation.  The most recent departures from that are the G36 series, and the UMP submachine gun.  These two weapon systems are the most overt move on the part of the engineers to produce a less expensive weapon, to be more competitive with other weapon producers.

The roller system is a work of art, and has its roots in the ashes of World War II.  The StG45 uses roller locking as did the MG42.   Refined first in the G3 rifle, it was the design of Mauser engineer Ludwig Vorgrimmler, used in the parental CETME rifle design, and perfected in the G3.  It does not seem necessary to go into a lengthy discussion of the roller system.  After studying the photos below, it should be understandable to the HK enthusiast.

Below the roller photos are diagrams of the basic trigger mechanism for HK select fire arms using roller locking as an operational basis.   Study of these diagrams will produce a basic understanding of the function of HK select fire arms.

This page should in no way be used to illegally convert a firearm to full automatic.  The diagrams were taken from a G3 manual, and are not a how-to for conversion.  However, HKPRO is on record as only for the compliance with law, and it is a federal crime to convert a firearm to automatic without a lengthy series of legal steps and licensure.  If you opt to break the law, you are risking a lot.  It is not worth it.

roller1.jpg (43716 bytes)

roller2.jpg (32498 bytes)

roller3.jpg (30435 bytes)
There are six points of tangency in the HK roller locking system:

A = Two entry points on rollers for energy.

D = barrel extension.  Receives 75 percent of the energy.  Points B and C are absolute maximum theoretical limits.

C would be no delay, straight blowback.

B means the gun would not open.

Points E receive the remaining 25 percent of the energy and create the necessary delay.


trig1.jpg (47859 bytes)
1. Bolt Carrier.   2. Firing Pin.  3. Release Lever.  4. Hammer Anvil.  5.   Trigger Spring.  6.  Hammer.  7.  Selector Lever.  8.   Hammer Shank and Spring.  9.  Catch.  10.  Elbow Spring and Roller.  11.  Sear.  12.  Trigger.


trig2.jpg (43101 bytes)
Only Selector Lever has moved from above photo.


trig3.jpg (20897 bytes)

Detail of Trigger Area


trig4.jpg (34622 bytes)

Trigger being pulled, semi auto mode, before hammer release.


trig5.jpg (15949 bytes)
Hammer at moment of release.


trig6.jpg (32401 bytes)

The moment of firing.


trig7.jpg (17198 bytes)
The moment after firing. (Trigger detail)


trig8.jpg (30656 bytes)

Bolt Rearward Movement


trig9.jpg (28307 bytes)

As hammer moves to rear, it is first caught by the sear, left, then the catch as it continues back.


trig10.jpg (23091 bytes)

Full Automatic fire.  Catch and bolt carrier are only mechanisms in play as trigger holds primary sear out of the way.