What is tolerance stacking and why is it critical when building/upgrading Glocks?

The Glock platform is one of the most versatile and widely-customized platforms to ever exist in the firearms industry. It’s design makes it approachable for the most novice of builders and it’s accuracy and reliability appeal to experienced shooters alike. The options for aftermarket components and upgrades are nearly endless, but this wide array of possibilities is also the cause of one the platforms biggest drawbacks… Enter tolerance stacking.

Every part and component in a firearm is manufactured to a specified dimensional tolerance which is designed to work in concert with the other parts/components that they interface with. In a Glock, for example, the trigger bar must be bent at just the right angle so that it’s high enough to engage with the striker lug, but not so high that it causes a heavy trigger pull. The trigger housing must sit in the proper position in the frame so that the trigger bar sits at just the right height. The slide must interface properly with the frame rails; a little too high and that trigger bar-to-striker lug engagement is affected. There are dozens of relationships like this between parts within the Glock operating system that all have to function perfectly with one another. Once you begin introducing aftermarket parts into the system – especially from multiple different manufacturers – you run the risk that one or more of those parts could be made to a slightly different specification than the factory counterpart.

In many cases, aftermarket manufacturers (NineX19 included) will design their products to have slightly tighter tolerances than the OEM parts. This tighter tolerance provides increased consistency in barrel lock-up, trigger feel, etc. That’s where the improvements in accuracy and performance come from. When you start to tighten tolerances, you also get slightly less “forgiveness” in the system and some friction surfaces need to be broken in and mated to one another over the course of a few magazine’s worth of live-fire. It’s not uncommon for a new barrel or slide to feel a little tight or possibly hang up a bit when it’s first installed and the slide is racked by hand. In most cases, however, a single trip to the range and a box of basic plinking ammo gets everything playing nicely together and provides for 100% reliability moving forward.

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