Sig Sauer MPX Copperhead 9mm – Superb 3.5 in. Coyote Piston Pistol

Description

The Sig Sauer MPX Copperhead 9mm is the shortest, densest expression of the MPX platform: a 3.5-inch barrel, a 14.5-inch overall length, and a Pivoting Contour Brace that folds the whole thing down to something that fits in a backpack. It is legally a pistol, not a short-barrelled rifle, and that distinction matters more than any spec on this page. Part PMPX-3B-CH, UPC 798681599011.

How It Works

The Sig Sauer MPX Copperhead 9mm runs a short-stroke gas piston with a fully closed and locked rotating bolt. Almost every other 9mm carbine on the market is a simple blowback design, where a heavy bolt and a stiff spring are the only things holding the breech shut. Blowback guns are loud, harsh and dirty. The Sig Sauer MPX Copperhead 9mm locks the bolt like a rifle and bleeds gas to unlock it, which is why it is noticeably softer shooting than a blowback gun of the same weight.

That locked breech has a second benefit: the system is tuned rather than brute-forced, so the gun cycles reliably across a wide range of ammunition and handles a suppressor without being over-gassed. The barrel is 3.5 inches of carbon steel with an integrated muzzle brake and a 1:10 twist, which stabilises standard 115 and 124 grain loads as well as heavier 147 grain subsonics.

Controls on the Sig Sauer MPX Copperhead 9mm are fully ambidextrous. Selector, magazine release, charging handle and bolt release all work from either side, and the forend is alloy with an M1913 accessory rail. The trigger is a single-stage polished hard-coat unit. Weight is 4.5 pounds bare.

What It Is For

A 9mm AR pistol this size occupies a specific niche. It is too big to carry on your body and too short to be a carbine, which leaves truck, pack, boat and bedside duty. The Sig Sauer MPX Copperhead 9mm gives you carbine-style controls, a red dot mounting surface and 20 rounds on tap in a package barely longer than a full-size handgun with a suppressor fitted.

The Pivoting Contour Brace is the piece that makes it work. It swivels on a patented pivot so it can be collapsed against the receiver for storage and deployed against the forearm when shooting. Used as intended it steadies the Sig Sauer MPX Copperhead 9mm considerably compared with shooting a brace-less pistol at arm’s length.

Finish on the Sig Sauer MPX Copperhead 9mm is Cerakote Elite in coyote, which holds up better than standard Cerakote against solvents and abrasion. There are no sights fitted. The M1913 rail runs the length of the receiver so a red dot mounts anywhere along it, and with a 3.5-inch barrel a red dot is not optional if you want to hit anything past across-the-room distance.

• Part: PMPX-3B-CH, UPC 798681599011
• Caliber: 9mm Luger
• Barrel: 3.5 in. carbon steel, 1:10 twist, integrated muzzle brake
• Operating system: short-stroke gas piston, closed and locked rotating bolt
• Overall length: 14.5 in.
• Weight: 4.5 lb
• Brace: Pivoting Contour Brace with patented swivel
• Forend: alloy, M1913 accessory rail
• Trigger: single-stage polished hard-coat
• Controls: ambidextrous selector, magazine release, charging handle, bolt release
• Finish: Cerakote Elite, coyote
• Sights: none fitted
• Capacity: 20+1, one MPX magazine included
• Legal classification: pistol, not an NFA item as configured

Honest Caveats Before You Buy

Read this part. As sold, the Sig Sauer MPX Copperhead 9mm is a pistol with a brace, and it is not an NFA item. Fitting a shoulder stock to it would create a short-barrelled rifle, which requires an approved ATF Form 1 and a 200 dollar tax stamp before the work is done. Rules covering stabilising braces have also been through repeated federal litigation, and state law varies independently of federal law. Confirm the current position for your state before you buy, and speak to a firearms attorney rather than relying on a product page.

MAG5 Firearms is not an SOT and does not sell NFA items.

Ballistics next. A 3.5-inch barrel gives up a lot of velocity compared with a 16-inch 9mm carbine, roughly 200 to 300 feet per second depending on load, so expensive defensive ammunition may not expand the way its manufacturer intended. Test whatever you plan to rely on, and look for loads designed for short barrels.

Noise and flash are the practical cost. An integrated brake on a 3.5-inch barrel produces serious blast, and indoors without hearing protection it is genuinely punishing. This is the strongest argument for suppressing the Sig Sauer MPX Copperhead 9mm, which brings you back to the Form 4 paperwork described above.

Finally, price and magazines. At 2,199.99 dollars this costs more than most 9mm carbines, and it takes proprietary MPX magazines rather than Glock or AR-9 magazines. One 20-round magazine ships in the box. Budget for three or four more and for an optic before you consider the Sig Sauer MPX Copperhead 9mm complete.

Who it suits: buyers who want a locked-breech 9mm rather than a blowback one, owners looking for a compact package for vehicle or home use, and anyone planning a suppressed 9mm build who wants a gun designed for it from the start. The Sig Sauer MPX Copperhead 9mm is a specialist tool with specialist paperwork implications. MAG5 Firearms is a family-owned FFL in Collegeville, Pennsylvania, with transfers, cleaning and gunsmithing in-house by appointment.

Browse more firearms in stock at MAG5 Firearms, or read the manufacturer specifications on the SIG SAUER MPX page before you order.