How to Convert Your AR-15 from 5.56 NATO to .300 Blackout or 22LR

How to Convert Your AR-15 from 5.56 NATO to .300 Blackout

The AR-15’s modular design makes caliber conversion a straightforward and rewarding process. One of the most popular conversions is switching from 5.56 NATO to .300 AAC Blackout—a round designed for superior performance in short barrels and optimized for suppressed fire. Whether you're building for home defense, tactical use, or range fun, this guide will walk you through the process of converting your AR-15 safely and effectively.

Why Convert to .300 Blackout?

Subsonic and Supersonic Capability: .300 BLK can fire both subsonic (quiet) and supersonic (high-velocity) ammo.
Optimized for Suppressors: Designed to function reliably suppressed, especially in shorter barrels.
Minimal Parts Swap: Only the barrel needs to be changed—everything else remains the same.
Versatility: Great for close-quarters, self-defense, and short-range hunting.

Step-by-Step Guide to Conversion

Step 1: Safety First

Unload your firearm.
Clear the chamber and remove all ammunition from the work area.
Always follow firearm safety protocols.

Step 2: Disassemble the Upper Receiver

To access the barrel, you’ll need to disassemble your upper:

Use AR-15 takedown tools to separate the upper and lower.
Remove the handguard, gas block, and gas tube.
Use an armorer’s wrench and torque wrench to remove the 5.56 barrel.

Step 3: Install the .300 Blackout Barrel

Select a quality .300 Blackout barrel, ideally between 8" and 16" depending on your intended use.
Insert and align the new barrel into the upper receiver.
Reattach the barrel nut and torque it to mil-spec standards (30–80 ft-lbs).
Reinstall the gas block and gas tube—ensure proper alignment for reliable cycling.
Reinstall your handguard.

Step 4: Confirm BCG and Mag Compatibility

The bolt carrier group (BCG) for .300 BLK is the same as 5.56 NATO—no need to change it.
Standard AR-15 magazines also work with .300 BLK. However, consider labeling them to avoid dangerous cross-caliber mix-ups.

Step 5: Function Check

Reassemble the rifle and perform a function check.
Verify proper cycling, safety engagement, and trigger reset.
Dry fire and manually cycle a few dummy rounds if available.

Step 6: Range Test

Begin with supersonic loads for reliable cycling.
Test subsonic ammo, especially if using a suppressor, to ensure proper gas system function.
Adjust or tune the gas system if needed (some setups benefit from an adjustable gas block).

Additional Tips and Tools

Use a bench vise and upper receiver block to securely hold your rifle during barrel installation.
Clean all parts before reassembly using your gun cleaning kit.
Always verify ammo compatibility before loading—never fire 5.56 in a .300 Blackout barrel.

Converting Your AR-15 from 5.56 NATO to .22LR

Another great conversion for AR-15 owners is switching from 5.56 NATO to .22LR using a conversion kit. This setup is perfect for inexpensive range practice and training.

Why Convert to .22LR?

Affordable shooting: .22LR ammo is far cheaper than 5.56
Low recoil: Ideal for new shooters and skill-building
Simple conversion: No need to change the barrel or gas system

Steps to Convert:

Unload and Clear the Rifle – Always start by ensuring your firearm is safe.
Remove the 5.56 BCG – Open the upper receiver and take out the standard bolt carrier group.
Insert .22LR Conversion Kit – Slide the .22LR bolt conversion unit into the upper receiver in place of your BCG.
Use Dedicated .22LR Magazines – Load up mags designed specifically for .22LR in the AR platform.
Function Test – Cycle the action and dry fire to ensure smooth operation.
Head to the Range – Enjoy cheap, low-recoil training with your converted AR-15.

Tip: For best accuracy, consider a dedicated .22LR upper, though it’s not required for casual use.

Final Thoughts: Build Smart, Shoot Safe

Converting your AR-15 to .300 Blackout or 22LR opens the door to a whole new world of performance. Thanks to the platform’s mil-spec compatibility and modularity, this upgrade is easy to perform with basic AR-15 tools and takedown tools. Whether you're after suppressed shooting or more stopping power in a compact rifle, .300 BLK delivers impressive results.