Carbine Buffer Tube (Honed) Kit w/ End Plate & Castle Nut

This kit is for AR-15 builders who want a Mil-Spec carbine receiver extension with the small details done right. You get a 6-position tube plus the hardware that finishes the install cleanly: an end plate and castle nut.

The inside of the tube is honed, which is a finishing step aimed at smoother spring travel and a more refined feel during cycling—especially compared with rougher tubes that can amplify noise and friction.

Why this kit is worth it

• Mil-Spec diameter for compatibility with Mil-Spec carbine stocks

• 7075-T6 aluminum for strength where it matters

• Honed interior to support smoother spring movement

• 6-position adjustment for common AR-15 carbine setups

• Includes end plate + castle nut so you’re not piecing parts together

What’s included

• Carbine receiver extension (buffer tube)

• End plate

• Castle nut

Specs

• MPN: ARM621-C

• UPC: 810048492916

• Material: 7075-T6 aluminum

• Fitment: Mil-Spec diameter

• Positions: 6

• Finish: Hardcoat anodized (black)

Compatibility notes

Works with AR-15 pattern lowers that use a carbine-length Mil-Spec receiver extension. If your stock is “commercial diameter,” you’ll want the matching tube size instead.

This kit is tube hardware only. It does not replace your buffer or spring unless you’re upgrading those separately.

Build tip

If you’re chasing a quieter, smoother recoil cycle, this tube pairs well with a captured system or a quality spring and buffer setup—but keep your changes simple and tune one variable at a time.

FAQ

Is this Mil-Spec or commercial diameter?

This model is intended to be Mil-Spec diameter, which is the more common fitment for AR-15 carbine stocks.

What does “honed” mean on a buffer tube?

It refers to an interior finishing process meant to smooth the inside surface so the spring can move more consistently.

Does the kit include a buffer and spring?

No. This is the receiver extension hardware set (tube, end plate, castle nut).

How many stock positions does it have?

It’s a 6-position carbine tube.

Will this work on AR-10 or 9mm builds?

This page is for an AR-15 carbine receiver extension kit. AR-10 and some 9mm setups can use different standards, so match your platform’s spec before ordering.

What are the part number and UPC?

ARM621-C and 810048492916.