How a Faulty Receiver Can Ruin Your Trigger

By Kevin Muramatsu



An AR’s trigger pull is the result of several factors working together or against each other. To manage the best pull possible, every one of these must be considered.

We most commonly think of the following:

Hammer Spring: At least 75% of the pull weight traces back to the strength of the hammer spring.

Trigger Spring: While the trigger spring also affects the trigger pull, its impact is much more evident in the trigger reset. A heavier trigger return spring will increase the trigger pull slightly, but it will snap the trigger back forward with much more force than a lighter spring.

Component Geometry and Finish: The smooth feel of a well-installed and timed match trigger comes from two things: proper sear angle and a refined interface between the sear and hammer. Any machining marks will produce a rougher, jumpier or grittier feel than will a nicely smoothed and stoned surface. (For more, check out our video on trigger prep)

Yet, there is a less considered aspect involved in the installation of match trigger groups. This is the dimensional location and relationship of the trigger pin holes.

At some point, just about anyone who installs their own trigger upgrades runs into a dimensional snag in a receiver:

  • A tight fire control box might cause the trigger to rub on the inside walls of the lower
  • The trigger pin holes might be too tight or too loose
  • The safety/selector might be spongy, tight or not rotate a full 90° due to the detent track being cut imperfectly

Any of these problems above can make the installation of fire control parts frustrating. Yet, each is a relatively easy fix. The placement of the trigger pin holes is another matter.

Over the years, the shop staff at JP has come across every kind of receiver flaw, and hole placement problems were the worst. We saw trigger pin and hammer pin holes at the ends of the tolerance run-outs and holes just flat out not where they were supposed to be.

So, if you plan on installing a trigger, here’s what to watch out for.

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