How To Build A 300 Blackout Upper

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This guide goes over how to build a 300 Blackout upper for the AR15 platform. The process is exactly the same as building a upper for a 5.56 AR15 upper. 


The only difference is the barrel chamber and muzzle device. If this is your first time building a 300 Blackout upper from scratch. Here is what you need to build a high quality upper receiver:

Gather All The Parts You Need First

ar15 upper ejection port

You will need to following to complete a 300 Blackout upper receiver before putting on a lower receiver.

  • 300 Blackout chamber barrel (Wider M4 feed ramp)
  • 300 Blackout BCG (Same as 5.56 BCG)
  • Stripped AR15 upper receiver
  • Charging handle
  • Handguard (comes with barrel nut)
  • Muzzle device 7.62 or 308 size 
  • Gas block 0.750" diameter
  • Gas tube
  • Torque wrench
  • Vice block
  • Reaction rod
  • Small set of roll punch pins

Install The Barrel

Installing the barrel is easy, but it can be difficult if you don't have a sturdy workbench setup to torque the barrel nut. In that case, take it to a gunsmith or a local gun shop to get that installed.

When installing the barrel, please make sure that the barrel seamlessly match the feed ramp grooves on the upper receiver. Use a GO and NOGO headspace gauge to ensure the distance between the face of the breech and the base of the 300 Blackout cartridge is perfect.

Excessive headspace poses risks, as it may cause unsupported brass to rupture in a rifle action, resulting in rearward gas blowback similar to rocket exhaust. Ensuring your safety is our top priority.

If a rifle closes on a GO gage, it means the rifle chamber can accommodate any ammunition that falls within SAAMI (industry standard) specifications. However, if a rifle closes on a NOGO gage, it suggests that the rifle chamber might have excessive headspace. 

If you don't want the trouble to just build one upper, please check out our best 300 AR15 upper receivers

Assemble Gas Block

If your selected barrel isn't dimpled, please also ask the gunsmith to do so. This is a very important procedure to secure the gas block on the barrel. 

  1. To dimple the barrel, you need a dimple jig to on the barrel. 
  2. Secure the jig on a vice block
  3. Use the drill to dimple the barrel with just enough space for the set screw to lock up

If you are using a low profile gas block, please pin the gas tube to the gas block first. Be sure the end of the gas tube is cleared to insert through the upper receiver gas tube port.

Next, slide the gas block on the barrel. Always slide in the gas block before putting on the muzzle device otherwise, the gas block can't get through.

Finally, install the set screws to secure the gas block to the barrel with some blue loctite compound. The set screws should lock up with the dimpled spot.

Torque The Barrel Nut

Torquing the barrel nut requires the stripped upper receiver to stay in place without warping. In order to keep the receiver intact when torque is applied, you MUST use a barrel reaction rod to hold the barrel, receiver index post and the upper receiver in place. 

For this step, we highly recommend the Midwest Industries reaction rod because it has the tightest tolerance for any mil-spec receivers.

Next, follow the torque spec for the barrel nut provided by the handguard packaging. For best practice, they are usually around 40 - 50 foot pound.

If you are using a delta ring, please make sure to time the ring so that the gas tube can go through. The challenge is that delta ring torque spec may not always match the alignment, therefore we highly recommend going with a free float rail system featuring a tube style barrel nut.

Install The Muzzle Device

For 300 Blackout, you need a 308 size muzzle device. They are generally thread with 5/8" - 24 thread pitch.

Most 300 Blackout AR15 barrels are threaded for 5/8" - 24 pitch. So you are good to go.

Secure the barrel and the upper receiver using the reaction rod, and torque the muzzle device until the ports are exactly where they should be.

Some muzzle device require the user to use different size shims to time it correctly on the barrel. Once you find the right shims and combination, you are good to go.

Secure The Handguard

Simply slide the free float handguard on the receiver and bolt it down based on the barrel nut's footprint. Be sure that the upper receiver rail aligns with the top rail on the handguard.

Install The Bolt Carrier Group

Slide in the charging handle to the upper receiver half way, then slide in bolt carrier group right under, then fully seat it.

The charging handle should be above to move the bolt without any issue.

Finally, you can assemble the upper onto a lower receiver 


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