December 22, 2023
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The Mk12 Special Purpose Rifle (SPR) series stands out for how accurate and effective it proved in both Iraq and Afghanistan. The Mk 12 SPR was such a huge success because it was a complete system, including rifle, suppressor, magnified optic and improved ammunition—the Mk 262 Mod 1 cartridge loaded with a 77-grain Sierra OTM bullet. This designated marksman rifle was in military service from 2002 to 2017. There were three distinct Mk 12 variants: the Mod 0, Mod 1 and Mod H. SPRs were built for all U.S. Special Forces (SF) and were fielded by U.S. Navy SEALs, the U.S. Army’s Fifth Special Forces Group, Army Rangers and others. But the Mk12 design is 25 years old, so the question is how does this design hold up when compared to modern ARs? Is it a valid choice, or only of interest to those shooters who love “clone” builds?
There are a number of different companies which make Mk12 copies, and for this article I secured a Mk 12 Mod 1 clone from Centurion Arms. Except for a few tiny differences, this rifle is a faithful reproduction of the rifle used in the Middle East during the war on terror. First, a brief history of the Mk12 project. Around the year 2000, the SPR program grew out of the SOPMOD program, and the “RECCE” rifles (pronounced “recky” and short for reconnaissance) being fielded by some Navy SEALs and other SF troopers. SOPMOD stands for Special Operations Peculiar Modification kit, and it is an accessory system for M4s that offers improved optics, grips, handguards, etc. The SEAL Recce rifles were ARs with longer and/or heavier barrels than the M4, often free-floating inside commercially available handguards, topped by magnified optics. SEALs, and other SF troops, began fielding accurized ARs in the 1990s. The SPR program was run by NSWC (Naval Surface Warfare Center) Crane Division, in Indiana.
The Mk 12 SPR grew out of a desire for a rifle more accurate and with longer range than the M4, but shorter/lighter than the 7.62x51mm SR-25. The people behind the SPR program came up with a number of rigid specifications for this new rifle. First, the Mk 12 SPR would sport a heavy, 18-inch match stainless steel barrel. These barrels had a rifle-length gas system and a 1:7-inch twist. The barrel muzzles were threaded the standard 1/2x28 and tipped with a muzzle brake suppressor mount made by Ops Inc. (which still exists as AE, Allen Enterprises). This single chamber muzzle brake has a threaded exterior (with a thread protector) designed to work with their suppressor. Two inches behind the muzzle brake you’ll see a tapered collar on the barrel that is meant to stabilize the Ops Inc. Suppressor.
Mk 12 Mod 0 SPRs are distinctive in part because of their handguard. Some would say PRI’s round carbon fiber forearm was the perfect balance between strength and weight at the time, and soon it became popular with 3-Gun shooters and was spec’d for the original SPR. It added weight when compared to a GI polymer handguard, but it was far stronger. The PRI handguards are somewhat thick, and between the thickness and the heat resistance of the carbon fiber material, it is almost impossible to get the handguard too hot to hold. The original SPR wore a #38 Swan Sleeve from ARMS. This MIL STD 1913 rail attached to both the upper receiver and the handguard, providing one continuous rail surface, long enough not just for a daylight optic but for night vision optic in front of that, as well as a spot at the front for a PEQ (IR laser). It had a flip-up rear sight which worked with the PRI flip-up front sight.
The PRI Gas Buster charging handle, which diverted gases away from the shooter’s face, was specced for this gun, as it was built to be run suppressed. The Ergo grip was also spec’d by the military for the SPR. There are several different versions of the SPR. The original soon became known as the Mod 0. It sports the round PRI handguard. The Mk 12 SPR Mod 1 is identical to the Mod 0 but for the handguard and sights. The round PRI handguard was replaced by a long KAC quad rail which also free-floated the barrel. Also, the PRI flip-up sights were replaced by KAC models, and the front sight moved back onto the handguard. Other than that, the Mod 0 and Mod 1 guns were identical.
Centurion’s Mk12 Mod 1 sports one of their 18-inch stainless steel match, air-gauged barrels with a black Cerakote finish. The SPR barrel has its own special heavy profile. It has a 5.56 chamber, 1:7-inch twist and a rifle-length gas system. It has an OPS Inc. muzzle brake and collar for suppressor mounting. The gas block is from Centurion and meets the NWSC Crane spec and is pinned in place. The barrel free-floats inside a long Knight’s Armament quad rail, identical to that found on the original Mk12 Mod 1. An optional bipod mount is clamped to the front of the rail, and during testing I attached a Harris bipod to it, as did many soldiers who used the Mk12 in combat. The upper and lower receiver are standard forged models. The upper receiver has M4 feed ramps. The bolt carrier group is Mil-Spec. The PRI Gas Buster charging handle is an available upgrade from a Mil-Spec model, and that’s what is found in my sample. The pistol grip is the original Ergo rubber model, and the stock is a standard fixed A2.
When the SPR program was in its infancy and the R in SPR stood for Receiver, the SPR uppers were installed on older M16A1 or M4A1 lower receivers. However, standard GI triggers were not what you want in a precision rifle, and they were swapped out for aftermarket options. Officially, the KAC 2-stage precision trigger was chosen as the standard when the SPR became a complete rifle, but before and after that point, SF trooper were installing various improved aftermarket triggers, preferring (in later years) the Geissele SSF. In Centurion Arms’ Mk12 Mod 1, you’ll find their AST, Advanced Sniper Trigger. This is a two-stage trigger which provides a 4.5-pound pull and sports a nickel-boron coating.
Empty, with no magazine, this rifle weighs 8 pounds even, which is probably less than you might expect. That KAC quad rail isn’t nearly as heavy as it looks. I recently tested a Mk12 Mod 0 from Precision Reflex, and it weighed 10 ounces more, and most of that was due to the handguard and rail, although that rifle came with flip-up iron sights, which is something this model lacks. It is 37.75 inches long. To compare the Mod 0 with the Mod 1, the Mod 0 with its PRI handguard is distinctive, but it adds a lot of weight that you don’t necessarily need. The Mk12 Mod 1 is substantially longer and heavier than an M4, but that’s not what you should be comparing it to. The Mk12 is meant to be shorter and lighter than the KAC SR-25, and it is about 2 pounds lighter and 7 inches shorter. The 5.56, even with the improved Mk262 Mod 1 ammunition specifically designed for the Mk12, can’t compete with the 7.62x51mm (.308) at extended ranges, but it was a huge improvement in both accuracy and terminal performance over standard military FMJ. I’ve been told most guns in-country were doing 1 to 1.5 MOA, which meant they were combat effective out to at least 700 meters.
I tested the Centurion Arms Mk12 Mod 1 for accuracy, and if you check out the accompanying table, you’ll see it would do .75 MOA with ammunition it liked. I topped it with the new Leupold Mark5 HD 2-10X, which is an evolution of the mid-range variable power optics which were found on the original Mk12s. The gun shipped with one Magpul 30-round magazine and was completely reliable during my testing. Recoil was nearly non-existent—you could shoot this rifle all day. The question, then, is whether the Mk12 is still a valid choice as a “modern” precision AR-15. You can find rifles which look more modern; that quad rail is dated, as is the Ergo grip and A2 stock. The Ergo grip was spec’d for the Mk12 because it added meat under the web of your hand compared to the military A2 grip. This was before Magpul started making any stocks or pistol grips. While the strong ACE collapsible stock was specced for the Mk12, many of the rifles sported fixed stocks. An 18-inch barrel seems to be a good balance between accuracy, velocity and length, which is why it was chosen in the first place. That heavy barrel adds a bit of weight, but that meat acts as a heat sink, and you won’t see substantive zero shifts if you’re doing a lot of shooting. While the quad rail looks dated, compared to more modern M-LOK compatible handguards, it really isn’t heavy, and it does free-float the barrel, allowing you to get the maximum accuracy out of your rifle.
The Mk12 concept still seems to be a valid choice in a DMR, even if you aren’t interested in a “clone” rifle. The Tier 1 SF units are currently in love with their 18-inch GFRs (Geissele Fighting Rifles) chambered in 6mm ARC and topped with LPVOs or MPVOs. These rifles are, for all intents and purposes, Mk12 SPRs, just chambered in a different caliber. Yes, the 6mm ARC hits harder at every range than the 5.56x45mm NATO, but where it really begins to shine is beyond 600 yards, and most shots taken in Afghanistan were well inside that. The magazine issue hasn’t quite been sorted out for this new 6mm ARC caliber, and 6mm ARC ammo is hard to find and expensive. So, again, there are incrementally better, more modern choices, but at what cost? If the 5.56 NATO is sufficient to your wants and needs, the Mk12 SPR does absolutely everything you need a DMR to do while being a piece of history.
Centurion Arms Mk 12 Mod 1 SPR Specs
- Caliber: 5.56 NATO
- Weight: 8 lbs.
- Overall length: 37.75 in.
- Receiver: 7075 T6 forged aluminum
- Barrel: 18-in. Centurion air-gauged stainless steel rifle-length gas system, 1/7” twist Gas block:low profile, pinned and tabbed
- Muzzle device: Ops Inc. (AE) muzzle brake
- Stock: fixed A2
- Pistol Grip: Ergo rubber
- Handguard: KAC Mk12 Mod 1 FF RAS
- Charging handle: PRI Gas Buster (GI charging handle standard)
- Trigger: Centurion AST, two stage, 4.5 lbs (as tested)
- Sights: none
- Finish: anodized
- Accessories: 30-round magazine
- MSRP: $2385 ($2525 as tested with bipod adapter and PRI Gas Buster)
- Manufacturer: Centurion Arms
If you have any thoughts or comments on this article, we’d love to hear them. Email us at FirearmsNews@Outdoorsg.com.