Mk 12 SPR Clone Build Guide: Specs and Parts in Stock

Short answer: the Mk 12 Special Purpose Rifle is an 18-inch, rifle-gas AR-15 built around a stainless 1:7 twist match barrel, a free-float handguard, a 3-9x power scope, a Harris bipod and 77 grain ammunition. To clone one today, start with an 18-inch Mk 12 profile barrel or a PRI Mk 12 Mod 0 upper, add a 3-9x scope, a two-stage trigger and a bipod, and feed it 77 grain match. With the parts below, that came to about $2,600 on 30 October 2026, before your lower, rings and magazines.

The spec sheet

What the Mk 12 was

US special operations units used the Mk 12, developed at Naval Surface Warfare Center Crane to fill the gap between the M4 and the SR-25, from about 2002 until it was phased out around 2017. It is 37.5 inches overall, and public references list Mk 262 ammunition at about 2,750 fps. Public references disagree on some details, and real rifles varied, so a clone is an interpretation, not a single blueprint.

Barrel and gas

The original used a Douglas 18-inch stainless match barrel with a 1:7 twist. Centurion Arms sells an 18-inch 416R Mk 12 profile barrel with 1:7 twist and rifle gas. BCM's Mk 12 barrel is 18 inches with rifle gas, in 410 stainless with a 1:8 twist, so check it suits your bullets. Note the rifle-length gas: on most 18-inch barrels you would see mid-length or intermediate, so check our gas system chart. See also the barrel length guide.

Handguard

The Mod 0 used a PRI Gen III free-float tube, and the Mod 1 used KAC's M4 Match free-float rail. PRI's complete Mk 12 Mod 0 uppers are the easy route, though the one below is a Gen II. Knight's sells a Mk 12 Mod 1 RAS, but its listing says carbine or mid-length gas, so confirm fit with your barrel first. See free float vs drop-in.

Optic and mounts

Both versions used a Leupold TS-30A2 3-9x36 scope in ARMS #22 rings, medium height on the Mod 0 and high on the Mod 1. The civilian equivalent was the Leupold Mark 4 MR/T. Any quality 3-9x40 scope gives you the same magnification range; we show a Trijicon AccuPoint below. See how to choose a scope.

Trigger, stock and bipod

BCM's history of the rifle lists a KAC two-stage trigger as standard; a Geissele two-stage is a sensible stand-in. Stocks varied between A1, A2 and M4 style, so choose what fits you. The bipod was a Harris on an ARMS throw-lever mount; a Harris needs a sling stud or a rail adapter on a modern rail.

Ammunition

The rifle was built around Mk 262, a 77 grain open tip match load from Black Hills. Its commercial cousins are in our catalog below. A 1:7 twist is what you want for bullets that long. See the 5.56 ballistics chart.

Suppressor

The Mk 12 ran an OPS Inc. 12th Model suppressor, which went out of production in 2014; Allen Engineering's AEM5 succeeded it. If you want the look, see the best 5.56 suppressors and how to buy one.

Parts in stock, with today's prices

Lowest price across the stores we track:

Build it part by part in the AR-15 builder, or browse AR-15 barrels.

Frequently asked questions

What is a Mk 12 SPR?

A US special operations 18-inch AR-15 for longer shots with a scope, bipod and 77 grain ammunition.

What is the difference between Mk 12 Mod 0 and Mod 1?

Mainly the handguard: PRI's Gen III tube on the Mod 0 and KAC's M4 Match rail on the Mod 1, with different ring heights.

What barrel does a Mk 12 use?

An 18-inch stainless match barrel with a 1:7 twist and rifle gas, originally made by Douglas.

What ammunition does a Mk 12 use?

Mk 262, a 77 grain open tip match load. A commercial 77 grain match load is the closest you can usually buy.

How much does a Mk 12 clone cost?

About $2,600 with the parts above on 30 October 2026, before your lower, rings, adapter and magazines.