New SierraPapa Trigger Group

New SierraPapa Trigger Group

There are three major areas of concern in improving the operation of the CX4 trigger mechanism. The first area of concern is the large amount of slack in the trigger mechanism noticed between when you first squeeze the trigger and the point where the trigger starts to actually release the hammer. The second area of concern is the feeling of roughness and irregularity in the trigger movement during hammer release. The third area of concern is the strength of the hammer spring.

The first area of concern: Long trigger travel.

Examination of the CX4 trigger group shows that the trigger does not act directly on the hammer sear as is usual with many rifles. The trigger actuates the hammer via a linkage consisting of two pressed steel arms and several pivot points that interact to eventually release the hammer. This linkage mechanism is similar to the arrangement found in many pistol trigger groups. The several pivot points and "bends" in the linkage mechanism allow for an accumulation of tolerances that cause excessive slack in the CX4 trigger pull. Since the original CX4 trigger is non-adjustable this slack cannot be removed.

Please note that it is not possible to remove ALL of the slack in the CX4 trigger mechanism. The design of the trigger mechanism requires a certain amount of free movement to allow the mechanism to reset between shots.

We have removed as much of the excessive trigger travel as is practical by supplying our adjustable aluminum trigger replacement.

The second area of concern: Rough hammer release

This problem of rough hammer release is caused by imprecise molding of the original plastic hammer and, to add confusion, the quality of the molding seems to vary from unit to unit which explains why some CX4s have reasonable trigger operation but others have very poor triggers. We have removed the problems with this part by supplying our stainless steel hammer replacement.

The third area of concern: Too powerful hammer spring

Since the original hammer of the CX4 is made of lightweight plastic it has very low inertia. This lack of inertia requires that the hammer be driven by a powerful hammer spring to ensure sufficient energy to reliably operate the firing pin. Unfortunately, in the CX4, the strength of the hammer spring has a great effect on the weight of the trigger pull. The result is that the powerful hammer spring required by the original hammer also causes an unnecessary increase in the weight of the trigger pull. The following statement may seem counter-intuitive but, since our stainless steel hammer is heavier than the original hammer (hence more inertia) we can achieve reliable operation with a less powerful hammer spring thereby reducing the trigger pull effort.


The overall problem with the CX4 trigger mechanism arises from three separate areas. Each of the three upgrade components we supply deals with a specific area of the problem. Installing our stainless steel hammer or our adjustable aluminum trigger alone will solve part of the trigger problem. We recommend that you fit the hammer, trigger and hammer spring modifications together for best overall results. We do not recommend fitting the reduced power hammer spring alone as this may result in unreliable ignition if used with the original hammer.