Des Moines, Iowa – -(Ammoland.com)- Before you begin make sure the gun is unloaded and completely empty. Be sure to mount upper receiver in vise block to provide a secure and steady hold without marring the finish.
Vise block also minimizes the possibility of damage to barrel detent pin and receiver detent notch. Prepare to disassemble barrel from receiver by removing all parts from barrel including flash suppressor, sight riser, gas block, gas tube, and forend.
- Use a good quality armorer’s tool with barrel nut wrench to loosen the barrel nut. Maintain light pressure on the tool in the direction of the receiver to prevent slippage. Unscrew the barrel nut and remove the barrel.
- Place new barrel into receiver and index into position with locating notch. Install new barrel nut and tighten by hand.
- Use a torque wrench attached to the armorer’s tool to begin tightening barrel nut with an initial setting of 30 pounds (the minimum setting) – again with light pressure on the tool toward the receiver. Increase torque setting by small amount, (10 pounds for example) up to 80 pounds maximum and continue tightening until gas tube port on the barrel nut lines up precisely with receiver gas tube port.
- The gas tube can be used to test alignment of the two ports. We recommend the Brownells Barrel Nut Alignment Gauge. Stop tightening when the two ports are precisely lined up. Double check that gas tube fits easily through the aligned ports. Misalignment at this point can affect accuracy as well as function. Leave gas tube in place. Install free-float handguard by turning until hand tight. Use a strap wrench if necessary.
- Maintain control of the gas tube with one hand and slide the front sight block assembly onto the barrel. Be sure front end of gas tube properly starts in gas block port as you slide the gas block in place.
- Insert a small punch or keeper pin horizontally into the gas tube pin hole to temporarily keep gas tube properly aligned, and then tighten compact gas block clamp just enough to hold it in place.
- Remove assembly from vise and place left side down on padded work surface. Install standard gas tube cross pin. Can start with a small hammer to lock gas tube in place.
- Loosen vise and use a good quality level such as Brownells Cross Test model to level the receiver.
- With the level on the gas block carefully rotate gas block as needed to exactly match the level reading taken on the receiver. Re tighten vise, double check the settings and then tighten clamp screws to lock gas block in place.
- Remove from vise blocks and test the installation by installing the charging handle, and bolt carrier with bolt. Work bolt back and forth, and make sure bolt closes smoothly with no interference at gas tube. Check headspace with a no-go headspace gauge and with a go headspace gauge. When properly installed in the chamber, the no-go gauge should not allow the bolt carrier with bolt to fully close. The go headspace gauge should allow the bolt carrier with bolt to close.
You can view a comprehensive video of this discussion on the Brownell’s web site.
Brownells is the world’s largest supplier of firearm parts, gunsmithing tools, equipment and accessories. Stocking more than 30,000 items, the company supplies armorers, gunsmiths, and shooters worldwide. All of their products are backed by a 100% satisfaction, unconditional, lifetime guarantee. To order, or for more information, call 800-741-0015 or visit Brownells.com
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