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The CMMG MK4 Receiver Extension is one of those parts you don't think much about until you're deep in a build and realize the whole back half of your rifle hangs off it. This one keeps things simple and correct: a mil-spec, 6-position carbine-length buffer tube built to the same external dimensions and thread pattern that fit military-issued M4 and M16 rifles. That means it threads right onto any standard mil-spec AR-15 lower and opens the door to the widest selection of stocks and braces on the market.
CMMG machines this receiver extension from aluminum and finishes it with a matte black hard coat anodized surface. That anodizing is more than looks. It shrugs off corrosion and holds up to the constant back-and-forth of the buffer and spring cycling behind your bolt carrier group. The tube houses your buffer and action spring while doubling as the mounting point for your buttstock or pistol brace, so a clean, straight, properly sized tube matters more than people give it credit for.
Being mil-spec, this tube runs the smaller stock-end diameter of roughly 1.14 inches, which is the standard most modern stock and brace makers design around. If your stock is labeled mil-spec, it'll slide on snug with minimal wobble. Commercial-spec stocks are slightly wider and won't lock up the same way, so match your furniture to the tube and you're set. The 6-position adjustment lets you dial length of pull to fit your body, your gear, and whether you're running a plate carrier or shooting in a t-shirt at the range.
This is a straightforward, no-nonsense receiver extension from a company that builds complete rifles, so they know what a good tube should feel like. Pair it with a quality castle nut, end plate, buffer, and spring, and you've got a buffer system that'll cycle cleanly for the long haul. Whether you're finishing a first build or swapping out a tired old tube, the MK4 covers the fundamentals without any drama.
It's mil-spec, meaning it matches the external dimensions and thread pattern of a military M4-style receiver extension. The stock-end diameter runs around 1.14 inches, which is narrower than a commercial tube. If your stock or brace is labeled mil-spec, this is the tube you want.
Yes. Any stock or pistol brace built to mil-spec carbine dimensions slides right on and locks up snug. Commercial-spec stocks are slightly wider and tend to feel loose or wobbly, so stick with mil-spec furniture to get the tightest fit.
The MK4 is just the receiver extension itself. To finish the job you'll also need a castle nut, an end plate, a carbine buffer, and an action (recoil) spring, plus the stock or brace of your choice. If you'd rather grab it all at once, a complete buffer kit bundles those pieces together.
It threads onto any standard mil-spec AR-15 lower receiver, which covers the vast majority of AR-15 builds out there. The threads and diameter are cut to the accepted mil-spec standard, so compatibility is rarely an issue. Just confirm your lower is mil-spec and not a commercial pattern before you thread it on.
| UPC | 816422020676 |
|---|---|
| Manufacturer | |
| Manufacturer Part Number | 55CA9D7 |
| Product Type | Buffer Tube |
| Shipping Weight | 0.25 |
| Style | Extension |
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