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PRACTICAL ACCURACY TESTING
HK 91 ISSUE TRIGGER VS. WTS’ STANDARD & SET TRIGGERS

With COMPATIBLE ammunition, most high quality semi-auto rifles are capable of producing minute-of-angle groups (1” at 100 yards; 2” at 200 yards; 3” at 300 yards; etc.) under ideal shooting conditions where human error can be eliminated.

The purpose of trigger work is to enhance a shooter’s trigger control under adverse shooting conditions where his skill is more important that the rifle's mechanical accuracy.

Two of the most interesting and realistic demonstrations of this point were published in the April ‘84 and July ‘84 issues of FIREPOWER Magazine.

In the April ‘84 article, an HK 91 rifle was tested for practical accuracy when it utilized both an issue HK 91 trigger (10# pull weight and .075” of creep) and WTS’ Standard trigger work (6# pull-weight with .03” of creep).

In the 200 meter bench rest tests, with a 9X scope, WTS’ Standard trigger work AVERAGED 43% smaller 10-shot groups in the slow-fire tests and 64% smaller groups in the rapid-fire tests (10 shots in 25 seconds).

Practical accuracy tests were also conducted for 200 meters with iron sights to determine the smallest target that could be consistently hit, at least 80% of the time, when each trigger was used from the standing and prone shooting positions.

In the slow-fire tests from both positions, WTS’ Standard trigger work allowed the shooter to hit targets that were almost 40% SMALLER than those that could be hit with the “issue” trigger.

In the rapid fire tests from both positions (10 shots in 40 seconds), WTS’ Standard trigger work allowed the shooters to hit targets that were at least 60% SMALLER than those that could be hit with the same “issue” trigger.

In the July ‘84 article, the HK 91’s practical accuracy potential was tested with both the “issue” trigger and WTS’ Set trigger, which had a 2.5# pull weight with .015” of creep.

In these tests, practical accuracy was measured in terms of the number of hits that could be made on a rectangular steel plate from various distances.

When compared to the “issue” trigger, WTS’ Set trigger AVERAGED 80% more hits from the bipod position and 152% more hits from the standing position.

To give you an idea how these mechanical energy values are used to measure trigger control, this table compares the HK 91 trigger pull specifications tested in the FIREPOWER articles to those of bolt action rifle triggers that have been adjusted for specific types of shooting.

When the various grades of HK 91 triggers are compared against each other, the reasons for the “issue” trigger's relatively poor practical accuracy become apparent:

The “issue” HK trigger had to be moved 2.5 times as far as WTS’ Standard trigger (.075” vs. .030”) and required over four times as much physical effort (.750”# vs. .18”#) in order to cause hammer/sear disengagement.

When the various grades of HK 91 trigger work are compared to bolt action rifle triggers, they have only slightly higher mechanical energy requirements than the bolt action rifle triggers.

BOLT ACTION RIFLE TRIGGERS VS. WTS’ SEMI-AUTO TRIGGER WORK

In most cases, a direct comparison between the “feel” of bolt and semi-auto rifle triggers is like comparing “apples to oranges” because most semi-auto rifle trigger designs require relatively large amounts of movement BEFORE (slack) and AFTER (over-travel) hammer/sear disengagement.

TWO-STAGE trigger designs such as those on the Garand, M1A, Mini14, FN 49, etc. have two distinct stages of hammer/sear disengagement.

During the first stage of trigger movement, 3 to 6# of pressure is needed to move the trigger rearward and eliminate 80-90% of the trigger's original hammer/sear engagement.

The second stage of trigger movement requires an ADDITIONAL 1-2# of pressure to move the trigger far enough to eliminate the remaining 10-20% of hammer/sear engagement.

In terms of trigger control, two-stage triggers can best be described as heavy, manually operated, set triggers because of the relatively small amount of ADDITIONAL physical effort and movement that is needed to cause complete hammer/sear disengagement during the second stage of trigger movement.

SINGLE-STAGE triggers on rifles such as the AR-15, AR 180, FAL, FNC, HK 91/93/94, etc. have exactly the opposite characteristics of two-stage triggers.

During the first stage of trigger movement, about 1-2# of pressure must be exerted in order to remove trigger slack; and an ADDITIONAL 4-10# of pressure must be exerted during the second stage of trigger movement in order to eliminate 100% of the trigger's original hammer/sear engagement.

In order to make a VALID comparison of the degree of trigger control that is provided by these radically different types of triggers, their resistance (pull-weight) must be multiplied by their movement (creep).

This calculation provides the number of “inch-pounds” (“#) of MECHANICAL ENERGY (physical effort) that is needed to move the trigger far enough to cause hammer/sear disengagement DURING the second stage of trigger movement.

One other point that needs to be emphasized in making comparisons between triggers with radically different trigger pull specifications is the effect that trigger creep has on trigger control.

This Trigger Pull Specification Ranges Table contains the normal range of pull weight, creep and mechanical energy specifications that exist on various types of semi-auto rifle triggers.

In analyzing this table, three basic conclusions will be reached:

1.  Radically different combinations of pull weight and creep specifications can produce the same mechanical energy requirements.

For example, a 6# trigger with .03” of creep requires the same amount of mechanical energy (.18”#) as a 3# trigger with .06” of creep BECAUSE the 3# trigger must be moved farther in order to cause hammer/sear disengagement.

radically higher mechanical energy requirements because the trigger must be moved farther in order to cause hammer/sear disengagement.

3.  Relatively small increases in trigger creep produce radical increases in the amount of TIME that is needed to move equal pull weight triggers far enough to cause hammer/sear disengagement.

For example, a 4# trigger's mechanical energy requirements will increase 25% if its creep specifications are increased from .03” to .04”.

For example, a 4# trigger with .04” of creep requires 33% more time to cause hammer/sear disengagement than a 4# trigger with .03” of creep because the .04” trigger has to be moved 33% FARTHER.