Spec to Inspect: Gauging the AR-15
To be honest, we tried really hard to avoid writing anything about gauging. It is a controversial topic that most people completely misunderstand, misrepresent, or misappreciate. We don’t feel like dealing with whiney people who don’t know what they are talking about, but we also want to get the information out to those who are interested in learning.
The simple truth is that the civilian component market is flooded with imposters and liars. The only way to make sure you are getting what you think you are is to check it against the TDP (or buy from a manufacturer that either builds for the government or who has a commitment to the TDP).
We are students of Chad Albrecht at School of the American Rifle (SOTAR). We also happen to have backgrounds in science, manufacturing, metrology, and quality, in highly regulated industries. So we understand and appreciate the concepts and value of specifications and quality control.
The TDP is our North Star. It sets the standard for the manufacture of AR-style arms for the U.S. Military. Standardization relies on specifications. Specifications are designed into manufacturing process controls in order to attempt to make something that conforms to a standard. An actual product is checked against specifications through inspection, which includes measuring and gauging. If you don’t appreciate this today, you don’t understand manufacturing and quality control. And that is fine. We hope we can help build your understanding and appreciation.
If you do understand this and still dismiss the importance of gauging, 1) you really don’t understand, and 2) you don’t realize that, unless you buy something that was manufactured for (and accepted by) the government, you are the only thing holding the manufacturer accountable. We hope we can change your mind.
For those who fully appreciate the value of quality control, we humbly submit to you our take on the technical inspection of the AR. We hope that it compliments your current understanding and appreciation.
Note that this series is no substitute for hands on training by someone who knows what they are doing. You will find no better AR armorer and gunsmithing course than Chad’s SOTAR AR-15 Master Armorer Course.
Quality is the basis for quality control and inspection. In this article, we introduce the governing principles of quality, and drill down to the principles of measuring and gauging. This will tee up the subsequent articles, in which we will explore the related concepts in greater detail.
Here we introduce the principles of geometric dimensions and tolerance, which together comprise the practice of Geometric Dimensioning and Tolerancing (GD&T). These concepts are used for communicating engineering specifications in a standardized way. They are the frame of reference for measuring and gauging and are used in the standard for the AR: The TDP. In this article, we explore these concepts and highlight the features of the TDP that we will use for our quality control activities.
Accuracy and precision are principles that most shooters have some grasp on. They are very important concepts in engineering, manufacturing, and quality control. In the article, we level set on the meaning of precision and accuracy, which will be foundational to our next few discussions.
In this article, we explore the practice of measuring. We look at the meaning of the practice, special considerations for instrumentation, what to do with the data, and our recommendations for instruments used to measure the AR.
In this article, we explore the practice of gauging. Gauging is the sibling of measuring, and brings consistency, objectivity, and speed to the quality control process. We look at the meaning of the practice, several special considerations for gauge selection and use, and our recommendations for gauges used to measure the AR.
We draw most of our expectations from the TDP. The full TDP is hard to come by as a non-Government contractor. Most commercial manufacturers have exploited this fact and have taken a lot of shortcuts while still asserting Mil-Spec conformance. With the TDP in hand, the information from the preceding articles, and, ideally, some hands on training, you can judge these manufacturers for yourself. Here, we offer what we believe to be the most complete published civilian collection of TDP drawings for the M16/M4 (for now, anyway).
For those interested in how we evaluate our components, this article is for you. Here we explain the specs that we use for measuring and gauging our components and why we use them.
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