The .284 Winchester is an example of a commercially unsuccessful cartridge that has enjoyed a resurgence due to interest from long-range competitive shooters. Introduced by Winchester in 1963, the .284 Winchester was designed to squeeze .270 Winchester and .280 Remington performance from the new Winchester Model 100 autoloader and Winchester Model 88 lever action rifles.
The end result was a 7 mm cartridge with about the same overall length as the .308 Winchester but with a wider body that yields a powder capacity about the same as that of the .270 Winchester and .280 Remington.
At one time the Savage Model 99 was available in .284 Winchester, and Ruger produced a small run of Ruger M77 rifles in this caliber, whereas Ultra Light Arms now builds more Model 20 rifles in .284 Winchester than all other calibers combined.
The .284 Winchester has 4.29 ml (66 grains H2O) cartridge case capacity. The case has a rebated rim and a body almost as large in diameter as that of typical belted magnum cases.
.284 Winchester maximum C.I.P. cartridge dimensions. All sizes in millimeters (mm).
Americans would define the shoulder angle at alpha/2 = 35 degrees. The common rifling twist rate for this cartridge is 254 mm (1 in 10 in), 6 grooves, Ø lands = 7.00 mm, Ø grooves = 7.19 mm, land width = 2.79 mm and the primer type is large rifle.
According to the official C.I.P. guidelines the .284 Winchester case can handle up to 440 MPa (63,816 psi) piezo pressure. In C.I.P. regulated countries every rifle cartridge combo has to be proofed at 125% of this maximum C.I.P. pressure to certify for sale to consumers.
The SAAMI pressure limit for the .284 Winchester is set at 56,000 PSI, piezo pressure.
When the cartridge over all length is maintained, deeper-seating is necessary with long heavier bullets. This reduces usable powder capacity and hence performance compared to longer cartridges like the 280 Remington.
The American .280 Remington cartridge is probably the closest ballistic twin of the .284 Winchester. When compared to the .284 Winchester the .280 Remington has a slightly different maximum allowed chamber pressure and case capacity.
For open country hunting of deer and pronghorn, the .284 Winchester loaded with the Speer 130-grain (8.4 g) spitzer at 3,100 ft/s (940 m/s) will do anything the .270 Winchester will do and it will do it in a short action rifle. Larger game calls for bullets weighing from 150 to 160 grains (10 g). H4831, H450, H4350, H414, IMR-4350, and IMR-4831 are excellent powders for the .284 Winchester.
These ballistics make it clear that the .284 Winchester is as good as the .280 Remington with the same weight bullet. Of course the short, handy mountain rifles for which the .284 Winchester seems best suited seldom come with 24-inch barrels. Aside from Winchester, no other major company has ever loaded factory ammunition for the .284 Winchester.
The cartridge is sometimes used for long range target shooting like F-Class and 1000 yd/m long range competitions, where participants usually handload their ammunition. For this application the .284 Winchester is loaded with 175 and 180 gr very-low-drag bullets.
The .284 Winchester is not popular in Europe, where it competes with the 7×64mm, to which it is almost ballistically identical. When compared to the .284 Winchester the 7×64mm has a lower C.I.P. maximum allowed chamber pressure and as a European 7 mm cartridge has a slightly larger bore. European 7 mm cartridges all have 7.24 mm (0.285 in) grooves Ø diameter. American 7 mm cartridges have 7.21 mm (0.284 in) grooves Ø.
While it has been occasionally factory chambered in various rifles, the chief reason for its survival has always been wildcatting. Wildcats are not governed by C.I.P. or SAAMI rules so wildcatters can capitalize on achievable high operating pressures. With the .284 Winchester as the parent case wildcatters have created 6mm-284, 6.5mm-284, .284 Shehane, .30-284, .338-284,35-284, .450 Bushmaster and the .375-284 variants and the .475 Wildey Magnum pistol cartridge.
Today, the most popular and useful .284 Winchester-case based cartridge is not the original, but rather the 6.5-284 Norma. This former wildcat was developed for long range target shooting where participants usually handload their ammunition. It is currently one of the most used non-wildcat cartridges by match shooters in F-Class and 1000 yd/m benchrest long range competitions.
Many owners of old Swiss service rifles in the United States are also now reforming .284 Winchester cartridge cases up to produce analogues to the more expensive 7.5×55mm Swiss GP11 cartridge.