The .277 Wolverine (6.8x39mm) is a wildcat cartridge developed and marketed by Mark Kexel, President of Mad Dog Weapon Systems (MDWS). It is a multi-purpose mid-power cartridge with increased ballistic performance over the AR-15's traditional .223 Remington (5.56×45mm NATO) cartridge requiring only a new barrel to upgrade/convert any 5.56-based firearm to .277 Wolverine.
For fundamentally the same reasons that Remington (et al.) chose a .277 inch (6.8mm) diameter bullet for the 6.8mm Remington SPC (i.e., ideal mass-to-diameter-to-length for mid-weight bullets constrained to loading in an AR-15/M16 STANAG magazine), MDWS selected 6.8mm as the basis for a new AR-15 wildcat. Unlike the 6.8 SPC and some other "larger bore" AR-15 cartridges (e.g., 6.5 Grendel, 7.62x39mm, .30 Rem AR, .30 Herrett AR, .450 Bushmaster, .458 SOCOM, .50 Beowulf, etc.), the .277 Wolverine uses a "standard" 5.56x45mm-based parent case – therefore rifle components such as the bolt and magazine are interchangeable between 5.56x45mm and .277 Wolverine firearms. A new barrel is the minimum required component to convert a standard AR-15 to .277 Wolverine.
In order to load heavier (therefore longer) bullets to magazine length without the problems of seating the bullet’s ogive into the case mouth, the Wolverine case is shortened to approximately 39mm from its 45mm parent brass. The case is resized and formed in a single-step operation to create new 23-degree shoulder and larger neck.
Initial design focused on optimal performance with supersonic bullets in the 85-115 gr class, therefore a 1:11 twist rate barrel with 5R rifling was selected. Subsequent consumer interest in firing "heavy-for-caliber" subsonic bullets led to the design, testing, and production of 1:7 twist barrels to stabilize the longer heavier bullets.
Initially developed, tested, and marketed as a proprietary cartridge, MDWS publicly released the detailed chamber and headspace gage specifications/drawings for the .277 Wolverine in June 2015. As of this posting, the .277 Wolverine has not yet been submitted to Sporting Arms and Ammunition Manufacturers' Institute (SAAMI) for standardization.
At least four commercial manufacturers have produced .277 Wolverine barrels, mostly for AR-15 conversions – although MDWS also sells barrels for Savage and Remington bolt-action rifles. Barrel manufacturers include: X-Caliber, McGowen, AR Precision, and PAC-NOR. By the end of April 2016, more than 1,000 .277 Wolverine barrels have been sold. Barrels lengths include: 8.2, 10.5, 12.5, and 14.5 inch pistol; 16, 18, and 20 inch rifle.
.277 Wolverine cases are easily made by the hobbyist reloader from plentiful and inexpensive 5.56x45 brass (shortened and resized). Extensive reloading data (including chronographed velocity, accuracy, and ballistic gel testing) is available on the Wildcat Shooters Forum for a wide range of bullet weights, commercially available smokeless powders, and barrel lengths. Reloading dies and tools are available from Hornady, Lee Precision, Sheridan Engineering, CH4D, and Little Crow Gun Works. The fire-formed .277 Wolverine case holds approximately 27.5 gr of water, compared to 28.5 gr for the parent case.
The .277 Wolverine has shown competitive performance to the 6.8 SPC with 110 gr bullets, achieving similar muzzle velocity (2,500 vs. 2,700 fps). The smaller case of the .277 Wolverine (compared to the 6.8 SPC) yields increased efficiency and therefore, less recoil (burning less propellant to achieve the same velocity with the same weight projectile produces less recoil). With lighter bullets (80-90 gr), although slower than typical 5.56×45mm rounds, the .277 Wolverine provides substantially increased energy due to their greater mass. For example, 60-62 gr bullets from a 5.56×45mm round typically provide less than 1,200 ft·lbs of energy, while 85-90 gr bullets from a .277 Wolverine round provide over 1,500 ft·lbs of energy (both at the muzzle of a 16-inch barrel).
There are always trade-offs between cartridge size, bullet diameter and weight, muzzle velocity, and energy on-target (at any given range). The .277 Wolverine will never replace larger high-power cartridges such as the .308 Winchester (7.62×51mm NATO) or .270 Winchester for long range shooting; however – it outperforms the .223 Remington at typical hunting ranges and is comparable to the 6.8 SPC while using less expensive components (brass, magazines, bolt).
The following table provides comparative performance data for several factory cartridges in the mid-power AR-15 class:
The .277 Wolverine provides similar ballistic performance to the 6.8 SPC for accuracy, hunting (varmints to medium-game), target shooting, and home/personal defense. 277/6.8mm bullets are widely available from virtually all major and many boutique reloading component manufactures (e.g., Speer, Sierra, Hornady, Nosler, Barnes, Remington, Winchester, Woodleigh, Lehigh Defense, Hawk, etc.) in a wide range of bullet weights from 85 to 200 grains and styles useful in the Wolverine - including lead-free practice, competition target, varmint, defensive, and game hunting. As proven by the venerable .270 Winchester and the newer 6.8 SPC, bullets in .277 caliber are capable of both outstanding accuracy and terminal performance. The Wildcat Shooters Forum documents (photographs and articles) numerous hunting successes ranging from 400-pound wild boar, to predators/varmints, to large northern whitetail deer.
Due to the modularity and flexibility of the AR-15, many .277 Wolverine owners simply build (or purchase) an additional barreled upper receiver for their 5.56x45 rifle. The complete bolt carrier group (carrier, bolt, firing pin, and cam pin) and charging handle can be swapped between the original 5.56x45 and the .277 Wolverine upper. The .277 Wolverine offers the AR-15 owner an alternative to obtain improved terminal performance over its parent 5.56x45 cartridge while minimizing cost of converting components and reloading ammunition.
A very wide range of cartridges from diminutive .17 rim-fire to massive .50 caliber rounds have been used in AR-15 rifles. The .277 Wolverine provides medium bore (.277") performance exceeding the parent 5.56x45 cartridge and the 300 BLK while approaching the power of the larger 6.8 SPC which requires larger cases and dedicated components (e.g., bolt and magazines). Per an interview with the cartridge's designer: "I liked the performance of the 6.8 SPC and the versatility of the 300 Blackout. After researching both of them extensively, and having a lot of conversations with many, many experienced professionals and enthusiasts, I decided to move forward with a new cartridge design that would hopefully combine the attributes of both, and dismiss the deficiencies."