One popular AR15 chamber conversion is switching from the standard .223/5.56 caliber to the highly effective .300 Blackout that is optimized for subsonic bullet shooting out of shorter barrels. In this expert guide, we'll walk you through the process.

300 Blackout Background

The .300 Blackout is a game-changer, offering a bridge between the popular .223 and 7.62 rounds. Developed by Advanced Armament Corporation (AAC) in collaboration with Remington Defense.

This round brings exceptional performance and compatibility to the AR-15 platform for shooting suppressed using subsonic ammo in the range of 190 to 230 grain.

Reasons to Use 300 Blackout In The AR15 Platform

300 blk rounds
  • Maintains compatibility with existing AR-style magazines, ensuring seamless transition and preserving magazine capacity.
  • Utilizes M4-style platform uppers and lowers, simplifying the conversion process.
  • Utilizes the same bolt carrier group as the 5.56 NATO AR-15 (including the extractor)
  • Optimized for suppressor use, delivering outstanding performance with subsonic ammunition.
  • Provides ballistic performance similar to the 7.62mm x 39mm round, enhancing stopping power and versatility.
  • Excels as a high-mass barrier-penetrating bullet, making it an ideal choice for short-barreled, suppressed configurations.
  • Method 1: Changing the Existing AR-15 Upper Receiver

    ar15 dirty upper receiver

    The simplest approach to converting your AR-15 to .300 Blackout is by swapping out the entire upper receiver. This method allows for a quick and straightforward transformation, saving you time and effort.

    Recommended Steps:

    1. Purchase a pre-built .300 Blackout upper receiver from a reliable source. Check out our selection of quality upper receivers here.
    2. Select an upper receiver optimized for your preferred barrel length. For optimal performance, a 9" barrel is recommended when using a suppressor. However, if you have a pistol AR-15 receiver, ensure that you pair it with a short barrel upper.
    3. Remove the existing upper receiver by separating it from the lower receiver. Refer to your AR-15 user manual for detailed instructions.
    4. Install the new .300 Blackout upper receiver onto the lower receiver, ensuring a secure fit.
    5. Confirm that all components are aligned and tightened properly.
    6. Test fire your converted AR-15 using .300 Blackout ammunition, and enjoy the enhanced performance!

    Method 2: Swapping the Barrel to .300 Blackout

    If you prefer a more hands-on approach and already possess the necessary tools, swapping the barrel on your existing upper receiver is an excellent option.

    This method allows you to retain your preferred upper receiver without having to spend a bunch of money.


    Recommended Steps:

    1. Acquire a high-quality .300 Blackout barrel compatible with your AR-15 upper receiver. Choose a reputable manufacturer to ensure reliability and performance.
    2. Gather specialized armorer tools, such as an armorer's wrench and a punch set, for the conversion process. Consider investing in a comprehensive armorer's kit to simplify future modifications.
    3. Safely disassemble your AR-15 by removing the handguard, gas block, and muzzle device.
    4. Using the appropriate tools, detach the .223 barrel from the upper receiver, taking care to follow proper procedures and guidelines.
    5. Align the new .300 Blackout barrel with the upper receiver, ensuring proper fit and alignment.
    6. Securely fasten the barrel to the upper receiver, following torque specifications provided by the manufacturer.
    7. Reinstall the handguard, gas block, and muzzle device, ensuring everything is tightened properly.
    8. Conduct a thorough function check and verify the alignment of all components.
    9. Test fire your converted AR-15 using .300 Blackout ammunition to appreciate its enhanced capabilities.