6mm ARC AR-15 Rock Vise Barrel Swap

Stumped by an AR-15 caliber changeover? In this video I secure a .350 Legend in the Rock Vise and convert it to 6mm ARC— step by step!

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About the Project

I’ve had my CMMG Resolute AR-15 in .350 Legend for a few years now and decided to convert the straight wall cartridge into something more versatile and suited to my needs: 6mm ARC.

.350 Legend on the Arrow Products Rock Vise

To make this change, I needed a 6mm ARC barrel, bolt carrier group (7.62 x 39 bolt face), gas tube, gas block, and the appropriate magazine.

6mm ARC Barrel, BCG, Gas Tube, and Gas Block

With the parts in hand, I just needed the appropriate tools to put it all together. I started with the Arrow Products Rock Vise and mag attachment block.

This accessory securely holds the lower in place in almost any orientation you could desire.

About Arrow Products Reaction Rods

Arrow Products offers two reaction rods: standard and production.

The standard AR-15 reaction rod is constructed of 7075 structural aluminum and has a milled ARCA foot. The production reaction rod has the same dimensions, 7075 structural aluminum construction, and milled ARCA foot, but with an A2 tool-steel patent-pending tip.

According to Arrow Products, this tip is “hardened to a Rockwell rating that allows for the ultimate in wearability while remaining ductile. The tip is secured to the shaft with six steel pins and a retaining bolt. The tip can be replaced as it approaches the end of its life cycle, while the rod can continue to serve an extended life.”

Making the Change

Replacing the mag attachment block with either reaction rod is simple — the rod ARCA foot connects directly to the ARCA jaw.

I then removed the bolt carrier group from the .350 legend upper and secured it to the reaction rod.

From here I could position the upper in any number of directions. I started by flipping it horizontally and removing the muzzle brake, then the handguard.

Note that the tools you need will vary depending upon your handguard, AR parts, and accessories. I keep a variety of tools on my bench, including a set or two from Wheeler’s new professional series.

These have a wide variety of neatly-organized bits. Next came loosening the barrel nut with a Wheeler professional armorer’s wrench and removing the gas tube and gas block.

(I had to use a mallet to help loosen the extremely tight barrel nut – the reaction rod held steady through all the torque!)

By this time, the upper receiver and barrel also stood alone. Reassembly with the new barrel is essentially a reverse of the process.

Screw the barrel nut onto the barrel and add the gas tube and gas block. When this is complete, fit this assembly into the upper receiver and secure it to the reaction rod.

This will allow you to torque the barrel nut and perform any other adjustments needed while the upper is held steady.

I made sure the bolt carrier group and gas tube were properly aligned before reinstalling the handguard. The last step was to properly clock and tighten the EC tuner brake.

Conclusion

Much of the appeal of AR-15’s come in their versatility and modularity. Changing a .350 legend to a 6mm ARC was very simple and easy — especially with the right tools. The rock vise and reaction rod securely held everything at whatever angle I needed to best make the switch.