How to Correctly Measure the Length of an AR 15 Barrel

The following is the method that the ATF uses to measure the barrel length of an AR 15 or variant at the time of writing this article.

First Things First

When preparing to measure the length of an AR-15 barrel, prioritizing safety is paramount. First and foremost, ensure the firearm is completely unloaded, removing the magazine and clearing the chamber of any rounds. It's wise to double-check this to eliminate any potential risks.

Opting for a clean, well-lit environment can enhance precision and safety during the measuring process. This provides clear visibility and minimizes the chances of any external factors affecting your measurements.

It's essential to understand that simply relying on a tape measure around the external contour of the barrel or referencing a stamped length on its surface may not provide an accurate reflection of the actual barrel length. For a precise measurement, measuring the bolt face to the end of the barrel is crucial, ensuring you account for every fraction of an inch. This will give you a definitive and accurate measurement, helping you stay compliant with legal specifications or requirements and informing your decisions on customization and accessories.

Remember, proper knowledge and understanding of your firearm's specifics are integral to its safe and effective use. Always measure with precision and caution.

How to Measure AR 15 Barrel Length

Before embarking on the process of measuring your AR-15 barrel length, it's paramount to ensure you're equipped with the right tools. At the very least, you'll need masking tape, a cleaning rod or a straight wooden dowel, and a reliable measuring tool like a tape measure or yardstick.

Start by inspecting the end of your barrel. It's important to determine if there's a muzzle brake or flash hider attached. If these are temporarily fixed, you'll want to remove them for the measurement. However, if they are permanently affixed, they become part of the barrel length in the eyes of the law, so leave them in place.

To accurately gauge your barrel's length:

1. Ensure the bolt is closed.
2. Gently insert the cleaning rod or wooden dowel down the barrel until it makes contact with the closed bolt.
3. At the point where the rod or dowel emerges from the barrel or permanently affixed muzzle device, wrap a small piece of masking tape around it. This tape will serve as a marker.
4. Carefully extract the rod or dowel, ensuring not to move the masking tape.
5. Lay it alongside your tape measure or yardstick to get the exact measurement.

Knowing the precise length of your barrel is not just about technical knowledge—it's a legal imperative. In the eyes of the ATF, a barrel even slightly under the 16-inch requirement can have significant legal ramifications. If you find your barrel verging on the shorter side, consider having a muzzle device permanently affixed to stay compliant with regulations.

Additionally, it's worth noting that measuring a detached barrel isn't the correct method. Once integrated into your upper receiver, the barrel's length might slightly differ. Ignorance of regulatory specifics offers no protection against legal consequences. Always prioritize accuracy and adherence to the law—it's always better to be certain than to risk non-compliance.

*A permanently attached device requires welding or pinning the device to the end of the barrel.