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Technical Engineering Detail of the G3

Technical Engineering Detail of the G3
The Roller Locking System in Detail



The G3 Automatic rifle has often been described in numerous German and foreign military and weapons journals.

Generally, the operation of the bolt system,  the basis of most other small arms of the HK weapons family was only treated briefly, and many times superficially.  This can be the cause of misunderstandings, especially if a delayed inertia bolt is spoken of.

As the leading manufacturer, we feel that the time has come to depict the bolt system of the G3 rifle in such a manner that, in addition to the circle of knowledgeable weapons experts, it can also be understood by technically interested weapons enthusiasts, without neglecting technical and physical accuracy.   Numerous tests confirm the validity of the theoretical principles described below.

Let us assume it is known that the simplicity and ruggedness of automatic weapon bolts, comprised only of a mass and a spring cannot be surpassed.

For this reason, our discussion should begin with this type of simple bolt system (Figure 1), especially since the bolt system of the G3 rifle represents nothing more than an elegantly designed modification of an inertia bolt.

In an inertia bolt, the projectile momentum, i.e. the impact of the expanding gases, is transferred to the bullet (2) in one direction and to the bolt mass (4) via the cartridge case (3) sliding out of the chamber, in the opposite direction.  The recoil spring (5) supports itself agains a mass (6) and returns the bolt mass to its initial position.

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Figure 1:   Transfer of impact to the bolt mass

The ratio between projectile and bolt velocity is just as simple as this of the major components arrangement.

At any moment during barrel time,

(bullet mass + 0.5  x load mass) x bullet velocity = bolt mass x bolt velocity.  The cartridge case obturation has been ignored in this case, as it is of no importance in considering the bolt system.

If a weapon with an inertia bolt were to be designed for 7.62 x 51 NATO ammunition, the bolt would have to weigh approximately 14 kg or 31 lbs.

To reduce this weight, much too high for practical purposes, without increasing the corresponding cartridge case extraction velocity (which could cause the cartridge to burst), the HK roller locked bolt is built in two sections.   In such a manner that during a certain time, the bolt head, driven by the cartridge case as it slides out, imparts a velocity greater than its own to the bolt head carrier, connected to it by a roller transmission.

Thus the bolt head drives the bolt head carrier by means of an interposed transmission.

The lever transmission is especially well suited for explaining the bolt system of the G3 rifle.

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Figure 2:   Transmission ratios in the G3 Bolt

The distance ratio a:b at the lever (6) is 1:4 in the G3 rifle, for example.  With reference to the receiver (9), the path of the bolt head carrier (7) quadruples with respect to that of the bolt head (5).   Because of the form locking connection between the two sections of the bolt, both sections travel their different distances in the same amount of time, i.e. during the duration of the effect of the impact (3), so that the velocities of both sections remain in the same ratio as the distance traveled.  Bolt head and bolt carrier move longitudinally within the receiver; the lever (6)must support itself in point A to provide the distance and/or velocity ratio between the bolt head and carrier.

Because of this support in the receiver, the impact, (3) transmitted to the bolt head (5) through the cartridge case (4)and occurring while the powder burns, affect the bullet (2) in one direction and simultaneously the bolt head carrier and the receiver in the opposite direction.  Meant by receiver or mass, are all parts which are rigidly connected with the receiver, e.g. barrel (1), and trigger assembly housing.

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Figure 3:   Momentum diagram in the G3 bolt.

If the lever ratio a:b = 1:4 is used as the distance and/or velocity ratio for the receiver mass m3, the ratio a:c = 1:3 applies for the distribution of momentum, i.e. 1/4 of the bolt head face momentum drives the bolt head carrier and the remaining 3/4 drives the receiver and the parts rigidly connected to it.

Applied to the bolt system of the G3 rifle, this means the following:

The velocity ratio R = a:b between bolt head and bolt carrier, characteristic of this rifle's bolt system, related to the receiver, permits a reduction of the bolt weight required in a pure inertia bolt to a value which results from the formula

Bolt weight


In order to drive the bolt carrier with the increased velocity during the unlocking period, the G3 bolt system does not have a lever, but an angular transmission, installed symetrically to the axis of the bore, with two rollers as transmission elements.

The ratio of the bolt head carrier with the locking piece travels backward four times faster than the bolt head, and only as long as the rollers move on the inclined surfaces of the barrel extension and locking piece.

Figure 4.  Ratio between bolt head and bolt head carrier with locking piece

During the last phase of the bolt's rearward travel, i.e. locking, the bolt head carrier forces the bolt head forward via the locking piece.  This causes the laterally projecting locking rollers to engage again.  the bolt head comes to rest at the cartridge base and the rollers against the cam surfaces of the locking piece.

Description of the design of the G3 rifle bolt

The vectors are drawn in with the actual angles and true to scale.  It is not possible to provide an exact value due to the unavoidable, so-called secondary gas effects, present in all automatic weapons.

In the discussion of momentum, (fig. 3), it was assumed tha the bolt head face momentum was equal to the projectile momentum.  However, this is only true in the case of straight cartridges.  In the case of a cartridge with a shoulder and a fluted chamber, the interrelations are somewhat more complex.  Of course, the momentum principle also remains valid in this case.

Force Vectors (Figs. 5-7):

The gas force (Max. 1.570 kg/460 lbs.) resulting from the burning powder acts upon both the bullet F4 and, within the cartridge case through the cartridge case base, the bolt head face.  In addition to this momentum, almost simultaneously a second impact, F5, is created which acts between the shoulder of the chamber andthe shoulder of the cartridge case because of the fluting in the chamber.  While the surface acted up0on by the gas is the entire shoulder surface, only the projection of this surface on the cartridge case base is of importance for the amount of momentum in the direction of the axis of the bore.  with the 7.62mm x 51 NATO cartridge, this surface happends to be the same as the caliber surface.

The bolt head force, F1, is transferred equally to both locking rollers.  the force acting upon each roller is again divided by means of the control angle 22.5 degrees and the recess angle of 50 degrees.   Force F2 acts upon the receiver in the direction of the buttstock, while F3 acts upon the bolt in the same direction.  Force F2 counteracts cartridge case shoulder force F5.  This produces the resulting force F6 (Fig. 7).  The trnasverse forces of F2 and F3 (Fig. 6) are absorbed by the receiver and the bolt.  The symmetrical arrangement prevents one-sided forces from occurring.

The recoil of the G3 rifle is comprised of two almost equally great impacts.  The receiver impact is followed after approx. 30 -40 ms by the buffer impact caused by the bolt.  Between them is the cocking phase of the recoil spring, which also exerts a pushing effect of approx. 10 kg (22 lbs.) upon the shoulder.  The overall recoil is therefore very well balanced.

F1 = Bolt Head Face Force F4 = Bullet Force
F2 = Receiver Force F5 = Cartridge Shoulder Force
F3 = Bolt Force F6 = Resulting Maximum Force


Figure 5: Diagram of forces


Diagram 6:  The Distribution of forces on the locking roller.


Figure 7:  Resulting maximum force accelerating the receiver towards the shoulder during gas development.
In summary, the following characteristics of the bolt system of the G3 rifle result from the previously mentioned interrelations:

1.  The advantages of the simple inertia bolt are retained, in particular the fact that only the pre-determined distribution of momentum and the area of the gas force curve throughout the time are of importance for the recoil velocity of the bolt.  this provides very good adaptability to all types of ammunition of the same caliber, bullet weight and velocity without the same adjusting elements in order to compensate for the shape of the gas force curve.

2.  The low extraction velocity most practical for the cartridge case is ensured by the physical and design principles, providing extremely secure cartridge support.

3.  The movement sequence of the bolt assembly and the receiver follows the gas pressure sequence without any delay.  the uniform, play-free commencement of all movements of the bolt parts and receiver avoids sudden, uncontrollable impacts.

4.  The bolt does not make any rotating or tilting movements when opening or closing.

5.  The bolt parts are arranged symetrically to the axis of the bore.  The roller contact poins are only at a small lateral distance from the axis of the bore.

6.  The sequence of the reaction force is uniform and without distinct peaks of force.

7.  Because the cartridge case pushes and does not have to be pulled, extractor strain is limited to ejection.

Points 3 - 6 contribute to the extremely high accuracy of the rifle.

It should be emphasized again that this type of bolt is a transmission with angular ratio which, on the one hand, considerably reduces the rearward movement of the bolt head to provide secure support for the cartridge case, and which, on the other hand, distributes the bolt head face momentum on bolt and receiver so favorably that the functioning parts receive optimum drive energy.

The friction occurring during the sequence of movements is negligible with respect to the bolt function in conjunction with the gas pressure sequence.

Cartridge case obturation, the greatest uncertainty factor in every automatic weapon, is reduced so much by the introduction and further development of the fluted chamber and has become uniform for all types of cartridge cases, including lacquered steel cases, that operation is reliable in every situation, even under the most adverse firing conditions.

Heckler & Koch, GmbH 1970

Note to the reader:  If you have gotten this far, then you obviously have some serious interest in the operating principles of the roller locked automatic firearms of HK.  For this reason, to tie this all together, I am linking the video of the G3 rifle firing on full automatic in ultra slow motion to this page.  It is at the top of the "Internal Operation" page, but is equally appropriate here.  If you made it this far, thanks, and let me know if you understand the technical jargon.  I sure don't, but I am not an engineer either.  I would appreciate hearing from someone who understands the principles here in detail.  This e-mail address is being protected from spambots. You need JavaScript enabled to view it