338 ARC Build Range Report

Published on October 3, 2025
Duration: 13:28

This video provides a range report for a custom 338 ARC AR-15 build by Brownells. It details the process of zeroing the rifle and adjusting a Rifle-Speed adjustable gas block for both subsonic and supersonic Hornady ammunition. The build demonstrates impressive accuracy at various distances, highlighting the potential of the 338 ARC cartridge.

Quick Summary

Adjusting an adjustable gas block involves ensuring reliable bolt lock-back on an empty chamber by increasing gas flow. Once achieved, open the gas block one additional slot to guarantee consistent cycling, especially as the firearm accumulates fouling. This process is critical for optimal performance with different ammunition types, like subsonic and supersonic rounds.

Frequently Asked Questions

How do you adjust an adjustable gas block for a new rifle build?

To adjust an adjustable gas block, first ensure the firearm is unloaded. Load a single round and fire it, observing if the bolt locks back. If not, increase the gas setting until it does. Then, open the gas block one additional slot for reliable cycling, especially as the firearm gets dirty.

What are the differences between subsonic and supersonic 338 ARC ammunition?

In this test, Hornady's 338 ARC subsonic rounds were 307 grains and supersonic were 175 grains. At 25 yards, their point of impact was nearly identical, but subsonic rounds required a more open gas block setting for proper cycling and extraction.

How accurate is the 338 ARC cartridge in an AR-15 platform?

The 338 ARC AR-15 build demonstrated impressive accuracy, achieving sub-MOA groups at 100 yards. The rifle was also capable of hitting steel targets at 200 yards, with subsonic rounds requiring about 1.5 MRAD of holdover due to significant drop.

What are the key considerations when tuning a gas block for different ammunition types?

When tuning a gas block, it's crucial to test both subsonic and supersonic ammunition separately. Subsonic rounds, due to lower pressure and velocity, often require a more open gas setting to ensure reliable bolt lock-back and extraction compared to their supersonic counterparts.

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