Note: Descriptions are shown in the official language in which they were submitted.
` W O 91/09263 ~ 0 '~ 72l1~ PCr/GB90/01969
MECHANISM FOR CON~ROLLING ~HE FIRING RATE
.
OF AN aU~OMA~IC WEAPON
This invention relates to a mechanism for controlling
the rate of fire of a weapon capable of automatic fire,
such as a sub-machine gun.
It is well-known that sub-machine guns have a tendency
to 'climb' during automatic fire. As each round is
fired, the weapon recoils upwardly and the muzzle tilts
to some degree and thereafter returns to the
approximate original polnt of aim. If the rate of fire
in fully-automatic firlng mode is too high, each
successlve round wlll be flred before the muzzle
returns on target followlng the discharge of the
preceding round and will therefore be high off target.
Each successive round will therefore be aimed higher
than its predecessor. The end result is what is known
as 'climb' - that is, a burst of rounds which start on
target and ~hereafter climb upwards off target. As the
rate of flre increases ln relation to a specific weapon
so wlll there be a tendency for the rate of 'climb' to
increase.
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It has been found that a rate of fire in the order of
400 to 600 rounds per minute can be reasonably
controlled by a trained user. At least to some extent
this controllability is dependent upon the design and
weight of the sub-machine gun; and also upon the
training of the user. Important factors contributing
to the rate of fire are the weight of the bolt and the
distance the bolt travels.
Previous attempts to reduce the size and weight of sub-
machine guns have usually necessitated a reduction in
the size and weight of the bolt and a reduction in the
distance the bolt is allowed to travel. Consequently,
this has resulted in a bolt with a faster action giving
lS weapons having rates of fire beyond the desirable
maxlmum rate of about 600 rounds per minute. These
efforts in size reductior. have therefore resulted in
weapons with varying degrees of climb and hence lack of
control.
A number of efforts to control the firing rate of a
sub-machine gun have involved increasing the mass of
the bolt and/or regulating the movement of the bolt.
In the first case the physical size of the weapon is
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reduced but the actual weight of the bolt is increased
by use of 'dense material' such as alloys of tungsten,
or infilling the bolt with lead. This method is self-
defeating in that the overall weight of the gun is
increased. Moreover, even the use of the densest
materials available gives only a minimal firing rate
reduction. Reducing a firing rate from l,800 RPM to
l,500 RPM falls well short of a reduction to a
controllable 400-600 RPM. Regulating the movement of
the bolt by 'lock-up' or inertia activated mechanisms
have involved utilizing the force generated by a
discharging cartridge and have the disadvantage of
disabling automatic firing of the gun if the mechanism
fails, and also have in practice not removed the rate
lS sufficiently to eliminate climb.
The mechanism of the present invention can be
incorporated in a sub-machine gun to give a weapon of
hand-gun size which has a controllable rate of fire in
the range of between 400 to 600 RPM. Furthermore, the
rate of fire is not predetermined by the inherent
physical properties of e.g. bolt weight or bolt travel
distance. The rate of fire is controlled by means
incorporated within the gun but independent of the
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force generated by a discharging cartridge and is
determined by test to suit each weapon in which a
mechanism according to the invention is incorporated.
Therefore, if a specific model of sub-machine gun
S firing in fully automatic firing mode returns to its
point of aim after each successive round is fired, say
in one-tenth of a second, then tha~ particular gun will
be adJusted to fire ten times per second or 600 RPM.
If another gun has, say, a longer barrel or lighter
frame, or has any other feature which changes the
firing characteristics of that gun, then that weapon's
rate of fire will be adjusted to a rate determined as
best sulted for achieving its maximum accuracy of fire.
lS The present invention relates to a trigger mechanism
which comprises a timing device wherein the
disadvantages of the prior art are overcome or at least
mitigated. The invention accordingly provides a
trigger mechanism for a gun capable of fully automatic
firing comprising a timing device operable to determine
the rat of fire of the gun, which device cyclically
interrupts means for discharging the ammunition to be
discharged from the gun when a physically displacable
trigger of the mechanism is moved to a firing position.
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According tQ a feature of this aspect of the invention,
said ammunition discharging means may comprise a
reciprocal bolt, said bolt being periodically arrested
to reduce its natural cyclic rate of movement during
automatic firing. Preferably, the device is an
electrically powered timing device which actuates a
main sear lever for controlling movement of the bolt.
According to a feature of the invention, the timing
device may be actuated by the trigger of the trigger
mechanism of the gun. In constructions where the
timing device is trigger actuated, the trigger
mechanism may comprise a trigger operatively connected
to a pair of sears each of which sears has a sear face
for co-operation with a bent, said bent being carried
by means for actuating a main sear lever for
controlling movement of the bolt. Preferably, said
pair of sears both co-operate with said bent when the
gun is set for semi-automatic firing, said bent
being released from the sear face of one sear when the
trigger is retracted to release the bolt from a cocked ,
disposition and being arrested by engagement with the
sear face of the other sear after discharge of the gun
but whilst the trigger remains retracted thereby
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to arrest movement of the bolt, said bent thereafter
being transferred into engagement with the sear face of
said one sear when the trigger is released to put the
gun once again into a cocked condition.
One sear may actuate the timing device and maintain the
device operative during automatic firing of the gun,
and the device may regulate movement of the said other
sear to cause a reduction in the natural cyclic rate of
movement of the bolt. Preferably, the timing device
incorporates a rotatable cam for cyclic engagement with
a cam follower carried by said other sear to regulate
movement of that sear.
The bolt and the main sear may have co-operating cam
surfaces which are engaged ~hen the bolt is retracted,
the main sear being pivotally mounted to release and
arrest said bolt and being controlled by a lever which
carries said bent.
Preferably, a firing mode selector switch is brought
into co-operation with said one sear to actuate said
device during fully automatic firing of the gun and is
taken out of co-operation with said one sear to
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deactivate said device during semi-automatic firing of
the gun.
Another aspect of the invention provides a gun
incorporating a trigger mechanism which comprises a
timing device as defined in any of the eight
immediately preceding paragraphs.
It is important to emphasise that a gun incorporating a
trigger mechanism according to the present invention
does not mean that the gun is an electrically fired
weapon. The firing rate of the gun is controllable by
electrical or electronic means in preferable
embodiments but should such means fail to function then
lS the gun is nevertheless fully operable in semi-
automatic or fully automatiC firing mode but, in such a
case, at the weapons natural cyclic rate of fire.
An embodiment of the invention will now be described,
2~ by way of example, with reference to the accompanying
drawings, in which: :
Figure l is a schematic side view partly shown in
section, of a sub-machine gun incorporating a trigger
mechanism and timing device according to the invention:
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Figures 2 to 7 are enlarged schematic and operationally
sequential views of the trigger mechanism and timing
device; and
Figure 8 is a plan view of a cam wheel of the timing
device and upper Qear lever.
Referrlng first to Flgure 1 of the drawings, a sub-
machine gun 10 has a breech 12 in which a bolt 14 (Fig
2 to 7) is reciprocable and slideable on a wear plate
surface of the breech which, in Figures 2 to 7, is
shown represented as a horizontal line 13.
Movement of the bolt indirectly is controlled by a main
!~
lS bolt sear lever 16 pivotally mounted at a leading end
thereof to the frame 'F' of the gun at pivot 20
disposed below the wear plate surface 13. At its
opposite trailing end lever 16 is pivotally mounted at
pivot 22 to a further lever 24 which carries a bent 25
for co-operation with a pair of let-off sears described
in more detail below. Lever 24 is pivotally mounted to
frame 'F' intermediate its upper and lower ends at
pivot 26. A trigger 28 is pivoted to the gun frame at
pivot 30, and is operatively connected to a lower sear
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WO91/09263 PCT/G~90/01969
32 of the pair of let-off sears by connecting link 34.
Link ~4 has one of its ends pivotally connected to
trigger 28 at pivot 36 and has its opposite end hinged
to the lower sear 32 at pivot 38. An upper sear 40
which co-operates with lower sear 32 to control
movement of the bent 25 is pivotally mounted together
with the lower sear to the frame at pivot 42. The
upper sear carries a cam follower 44 which co-operates
with an actuating cam of a timing unit 46 disposed in
the butt of the gun.
In Figures 2 to 7 of the drawings, the large arrows
indicate in each case the direction in which a
component of the trigger mechanism or firing rate
lS control device is urged to move either by spring force
or, in the case of the latter, by electrical propulsive
force or other actuating means.
Referring to Figure 2, the mechanism is shown in a
condition in which the gun is cocked and ready to fire.
In this condition, the bolt 14 is held retra~ted by the
main sear lever l6. The main sear lever l6 cannot
however, on its own, retain the bolt l~ in a retracted
position because the sear face 17 and bent l5 of the
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bolt each provide cooperating 45 degree cam surfaces
and because the main spring force indicated by the
arrow on bolt 14 is greater than the main sear return
spring force, shown by the arrow on lever 16, bolt 14
is urged forwards so that bent 15 is forced into
abutment with the sear face 17 of sear lever 16.
However, because the lever 16 is directly linked to
lever 24, if bent 25 of lever 24 is locked against
either the sear face 33 of sear 32 or sear face 41 of
sear 40, the main sear lever 16 cannot move, and
therefore retains the bolt 14 in its retracted
position. In Figure 2, bent 25 is shown held by the
lower sear 40 at sear face 41.
As previously stated the trigger 28 is directly linked
to the lower sear 40 by link arm~34. As can be seen in
Figure 2, if the trigger is pulled back ( retracted to
the right in Figure 2) then the lower sear 40 will be
caused to rotate clockwise (downwards) and thereby
disengage sear face 41 from bent 25 thereby allowing
lever 24 to pivot. Consequently, the bolt is then free
to over-ride the main bolt sear lever 16 and move
forwards (left in Figure 2) to discharge the gun.
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The upper ~et-off sear 32 has limited movement in a
clockwise direction. Upper sear 32 can move only until
face 33a of sear 32 meets face 41a of sear 41. Indeed,
sear 32 and sear 40 are biased against each other at
faces 33a and 41a by spring force between those sears
in the manner of a common sprung clothes pe~ as
lndlcated by the ~uxtaposed arrows. Pivot pin 38
protrudes axlally and acts upon one of three stepped
face~ S, F and A respectively of a firing mode selector
swltch 39 shown ln Figures 2-7 superimposed upon the
lever 24 and sear 32. When the firing mode selector
switch is in its lowermost position, face 'S' is
aligned with the path of movement of pivot pin 38
(Figure 2) whlch prevents plvotal movement of lower
sear 40. However, when flring, mode selector swltch 39
is put into its lntermedlated ,posltion, face 'F' ls
allgned wlth the path of movement of pivot pln 38 so
that limited pivoting movement of lower sear 40 can
take place. Thus, as the lower sear 40 is rotated by
retraction of the trigger 28 via link 34, the upper
sear 32 will also have a tendency to rotate in the same
direction as the lower sear 40. Sear face 33a can move
away from sear face 41a a distance sufficient to follow
bent 25 to pass between the faces 33a and 41a.
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It will be seen in Figure 2 that when the bent 25 is
engaged with the lower let-off sear face 41, bent 25 is
beneath sear face 33 of the upper sear 32.
A firing mode selector switch 39 shown in Figure 3 is
set to a semi-automatic firing mode, so that the switch
39 in this firing mode will allow movement of sear 40
until pivot pin 38 abuts face 'F' of firing mode
selector switch 39 but will not allow sufficient
movement of sear 40 for it to engage timing activator
switch 54. Face 'F' stops sear 40 just before it
engages switch 54 but just after sear 40 disengages
from bent 25.
In Figure 3, the trigger is shown as having been
pulled, thereby causing the sear face 41 of let-off
sear 40 to disengage from bent 25 of the lever 24.
Figure 3 shows events at the moment of disengagement.
It can also be seen that the upper sear 32 does not
move downwards, because it is held back in
position by bent 25.
Figure 4 shows the relative positions of the components
of the trigger mechanism immediately after the
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positions s~hown in Figure 3. Lever 24 is no longer
retained at bent 25 by let-off sear 32 and consequently
bolt 14 is able to over-ride the main reduction sear
16, thereby pushing the main sear lever 16 downward and
causing anti-clockwise rotation of lever 24 away from
engagement with elther let-off sears 32 or 40.
The main sear lever 16 must, of course, move below the
surface 13 of the wear plate to release the bolt 14.
Referring now to Figure 5, the trigger is shown still
in its pulled (retracted) position and the relative
positions of the trigger components are shown in
positions immediately after those shown in Figure 4.
The bolt 14 has now discharged a cartridge and has been
returned in reaction to discharge of the cartridge
along the breech allowing the main sear lever 16 to
snap upwards under spring bias into engagement with the
bolt at faces 15 and 17 to arrest forward travel of the
bolt. As the main sear 16 snaps upwards, it
simultaneously causes the lever 24 to rotate clockwise
and causes the bent 25 of lever 24 to slide under and
beyond the sear face 33 of the upper sear 32. Sear 32
. '
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becomes free to return to its neutral position against
the lower sear 40 in which faces 33a and 41a are in
abutment. In so doing, the upper sear 32 engages the
bent 25 at sear face 33 and arrests return
movement of lever 24 before the bolt 14 can over-ride
cam face engagement at faces 15 and 17. The bolt is
thereby locked in a retracted position.
In order for the gun to be fired again in semi-
automatic firing mode, the trigger must be released to
reset the mechanism. Thus, referring again to Figure
2, as the trigger 28 is released, it causes the lower
sear 40 to rotate anti-clockwise (upwards). This
action causes the upper sear 32 also to move upwardly
because of the engagement between the sears at faces
32a and 41a. As the upper sear 32 disengages from bent
25 at face 33, face 41 of the lower sear 40 moves into
position so that bent 25 is transferred into engagement
with face 41 of the lower sear. The weapon is then
fully cocked and the trigger is again ready to be
pulled.
It will be appreciated that if the upper sear 32 were
not present to engage bent 25 after the sear 40 had
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disengaged,~the bolt would be free to continue moving
back and forth after an initial round of ammunition had
been discharged from the gun. In full unregulated
automatic firing mode the weapon would continue to fire
S until the supply of ammunition was exhausted or until
the trigger was released thereby re-engaging the lower
sear 40 with the bent 25. In order to fire the gun in
regulated fully automatic firing mode, the selector
switch 39 is brought into engagement so that the
trigger mechanism activates timing mechanism 46 to
control movement of the bolt.
Referring now to Figure 6, the selector switch 39 is
shown in its uppermost position in which face 'A' is
lS aligned with the path of movement of pivot pin 38.
When selector switch 39 is in this position lower sear
40 can pivot sufficiently clockwise as to engage and
depress timing activator switch 54 when the lower sear
40 i~s activated via the trigger 28, thus activating the
timing unit 46. The timing unit comprises a motor (not
shown) which, when active, causes a cam wheel 56 to be - ~ -
rotated in a clockwise direction as seen from above
(see Figure 8).
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Other than ~the intervention of the selector switch 39
the various components of the mechanism are in the
relative positions as shown in Figure 5 for semi-
automatic firing. Now, however, after the trigger is
retracted and is maintained in pulled back position,
and after an inltlal round of ammunition has been
discharged, because the lower sear 40 has been rotated
out of engagement with the bent 25, and the upper sear
32 has re-engaged the bent 25, the lower sear 40 serves
only to hold switch 54 in a depressed position.
In Figure 8, the cam wheel 56 is shown in plan view.
Once per revolution of the cam wheel, a cam lug 58
carried by the cam wheel will engage-cam face 44 of the
lS upper sear 32 and cause the upper sear to rotate anti-
clockwise (upwardly) thereby disengaging from bent 25
at sear face 33. Each time the cam 58 causes the upper
sear 32 to disengage from bent 25, the gun will be
fired and thereafter the upper sear 32 will re-engage
bent 25. ~his will happen only in a cycle in which the
period of the cam is less than the natural firing
period of the weapon itself. As a safety feature, when
the gun is cocked, the cam lug cannot engage the cam -
face of the upper sear until the trigger is fully
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retracted. Therefore the movement of the components of
the mechanism can be arranged so that the gun can be
fired in semi-automatic mode with a less than full
trigger pull even with the automatic firing selector
switch engaged.
The upper sear 32 must have time to re-engage bent lO.
The firing rate can be decreased infinitely by
different gear trains or voltage changes etc, but the
firing rate cannot be increased beyond the gun's
natural un-regulated firing rate.
Figure 6 shows the mechanism in a cocked condition, and
Figure 7 shows the cam 56 positioned to dislodge the
upper sear 32 out of engagement with bent 25 of lever
24. This cycle of timed automatic firing will continue
until the supply of ammunition is exhausted or until
the trigger is released. Upon release, the trigger
allows the lower sear 40 to engage bent 25 of lever 24
- 20 and allows lower sear 40 to disengage from switch 54,
thereby de-activating the timing motor and cam wheel
56. ~he gun is then ready to be reloaded, if empty, or
fired again.
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The trigger~ mechanism is constructed and arranged to
reduce the let-off force at bent 25. If face 15 and 17
were perpendicular to the direction of bolt movement as
is normal in known weapons, a main spring force of e.g.
eighteen pounds would generate a sear let-off force of
around six pounds. However, because the bent and sear
faces of the bolt and lever 16 respectively must at a
45 degree angle to the perpendicular an eg. eighteen
pound mainspring force exerts only a nine pound
downward force on the main sear lever 16 and therefore
that force is transmitted to lever 24 at connecting
pivot 22. The bent 25 of lever 24 is twice the
distance away from pivot 26 as is pivot 22 so that the
force transmitted to bent 25 of lev~r 24 is one half of
that exerted on pivot 22 of lever 24. Therefore, a
nine pound force is reduced to a four and a half pound
force at bent 25 against sear face 33 or sear face 41.
The co-efficient of friction of a four and a half pound
force (depending upon the finish of the mating
surfaces) of bent 25 and sear faces 33 and 41 can be as
low as 0.5. The upper sear 32 is the active let-off
sear when the gun is set in regulated fully-automatic
firing mode, so notwithstanding the return spring force
of sear 32, the actual let-off force the timing unit
must overcome with may be one pound or less.
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In both semi-automatic and fully-automatic firing
modes, the lower sear 40 holds the bolt in a cocked
conditlon. Since the lower sear 40 is manually
controlled via trigger 28, the lower sear can have a
much heavier return spring.
It is envisaged that should one require an extremely
light trigger pull for semi-automatic firing then, for
extra accuracy, a system could be employed which
temporarily disengages the heavy trigger return spring
(for one shot), and then re-engages after the trigger
is released.
The forces holding let-off sears 32 and 40 can be
lS further reduced by substituting the return spring
acti~g on the main bolt sear 16 and the lever 24 with
one which exerts a greater force. The above example in
which an eighteen pound main spring force transmits a
nine pound downward force against pivot pin 22 occurs
only if there is no spring force acting on sear 16 to
urge it upwardly. If the return spring acting upwardly
on sear 16 has a force of e.g., four pounds, then the
actual force transmitted downwardly onto sear 16 and
pivot pin 22 is five pounds so that only a two and a
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half pound ~orce is exerted at bent 25. A main sear
return spring force of about nine pounds would negate
a nine pound downward force transmitted by the bolt
main spring, and the bolt would not then move when the
trigger was pulled.
Lever 24 has a portion above pivot pin 26 whose mass is
equal to the mass of the lever below the pivot pin 26.
Lever 24 is a elongate component positioned
perpendicular to the line of motion during recoil. The
presence of the portion above the pivot pin 26 cancels
out any oscillation in lever 24 which could adversely
affect the lock-up of bent 25 with let-off sear 32 or
40.
The above embodiment employs a cam wheel to trip the
upper sear 32. However, any device which delivers the
necessary force and can reliably and repeatedly trip
the upper sear 32 with the desired and consistent
period, could be utilized. The motion of such a device
could be reciprocal as opposed to rotary,
such as that produced by a solanoid.
The power source for activating the trip device could
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be other than electrical, such as mechanical, clockwork
or gas pressure. However the present embodiment has
the advantage that the timlng unit is a self-contained
unit, sealed against the elements. It has a life
expectancy of up to and in excess of the life of the
gun (i.e. 50,000 rounds) depending upon battery slze.
The timing unit may incorporate warning lights to
indicate battery condition.
The mechanism of the present invention ideally ls
adapted for incorporation in an open-bolt 9mm sub-
machine gun although a closed bolt may be utilized
simply by the replacement of one bolt type for the
other. This feature allows the gun to be used with a
closed bolt mainly in semi-automatic firing mode when
accuracy is of primary importance but where the option
of a fully automatic firing mode is available, perhaps
in an emergency, even with a closed bolt.
Should the timing unit fail then the gun normally can
be fired as a semi-automatic weapon. Alternatively it
is envisaged that the gun may be furnished with a
selector device (not shown) which engages the upper
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sear 32 and ~holds it up out of the path of travel of
bent 25. This facility allows the gun to be fired in
unregulated fully automatic firing mode.
S It is also envisaged that means such as an electric
spark may be utilized to discharge the ammunition
rather than a mechanical bolt in which case the timing
unit is adapted to control the emission of
the detonating spark.
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