History
The 6-284 Winchester is the .284 Winchester necked down to take .243 inch 6mm bullets. Winchester drew up the parent in 1963 around a deliberately fat, rebated-rim case: a .500 inch body on a .473 inch rim, sized to pack near-.270 powder capacity into a short action. Necked to 6mm, it handed wildcatters one of the highest-capacity 6mm cases going, and it anchored 1,000 yard benchrest and F-class for years before purpose-built cartridges arrived.
No SAAMI or C.I.P. specification stands behind it, so the dimensions here are derived from the parent .284 Winchester and should be treated as approximate. Shooters chase it for raw 6mm velocity with heavy match bullets, and the tradeoff is a thoroughly overbore case that burns throats fast. The 6mm Creedmoor and 6 Dasher now reach much the same speed on far longer barrel life, so the 6-284 has settled into heritage rather than mainstream.
Lineage
The 6-284 Winchester is the .284 Winchester case necked down to .243 inch (6.17mm), keeping the parent's rebated rim and its roughly 35 degree shoulder. It runs in a short or standard action on a large rifle primer. The parent is the .284 Winchester; the peers are the other high-capacity 6mm target rounds, the 6XC, the 6mm Creedmoor, and the 6 Dasher, the last of which lands in the same place without any wildcatting.
Specifications
| Spec | Value |
|---|---|
| Case type | Rimless rebated, bottlenecked (~35° shoulder) |
| Bullet diameter | 6.17 mm (.243 in) |
| Neck diameter | 7.09 mm (.279 in) |
| Shoulder diameter | 12.06 mm (.475 in) |
| Base diameter | 12.70 mm (.500 in) |
| Rim diameter | 12.01 mm (.473 in, rebated) |
| Case length | 55.12 mm (2.170 in) |
| Overall length | 71.12 mm (2.800 in) |
| Case capacity | ~66 gr H2O (nominal) |
| Primer size | Large rifle |
| Belted | No |
| Rifling twist | 1 in 7.5 in (recommended, for heavy 6mm match) |
| Max pressure | No SAAMI/C.I.P. standard; load to .284 Win pressure |
| Recommended barrel | 26 in, 1:7.5 twist |
Dimensions are derived from the parent .284 Winchester; the 6-284 is a wildcat with no published standard, so treat the figures as approximate. The case is rebated, a .473 inch rim on a .500 inch body, which is how a fat, high-capacity case still feeds through a standard .473 inch bolt face.
Barrel Design
The 1:7.5 twist is the call that defines the cartridge. The 6-284 exists to push heavy 105 to 115 grain 6mm match bullets at the top of what the bore can do, and bullets that long need a fast twist to stabilize. Build it with a hunting twist and you have thrown away the only reason to chamber a case this big in 6mm.
Being deeply overbore, it makes the most of a long barrel that lets the slow powders finish burning and tightens standard deviation on a match load. I'd put my baseline at 26 inches, which collects the velocity while still handling like a target rifle. The tradeoff is barrel life: a case this big over a 6mm bore erodes the throat fast, and match accuracy is measured in the low thousands of rounds, not the life of the rifle. The tables below are computed at that 26 inch barrel from representative handloads, and since there is no factory 6-284 ammunition, confirm your own on a chronograph before trusting any dial.
Handload Performance
Handload · 109 gr Berger Long Range Hybrid Target
| Range (yd) | Velocity (fps) | Elevation (mil) | Energy (ft-lb) | Windage (mil) |
|---|---|---|---|---|
| 0 | 3000 | -0.3 | 2178 | 0.3 |
| 100 | 2850 | 0.0 | 1966 | 0.1 |
| 200 | 2704 | -0.3 | 1770 | 0.3 |
| 300 | 2562 | -0.9 | 1589 | 0.5 |
| 400 | 2424 | -1.6 | 1422 | 0.6 |
| 500 | 2291 | -2.3 | 1270 | 0.8 |
| 600 | 2162 | -3.1 | 1131 | 1.0 |
| 700 | 2037 | -4.0 | 1004 | 1.2 |
| 800 | 1916 | -4.9 | 889 | 1.4 |
| 900 | 1799 | -5.9 | 783 | 1.6 |
| 1000 | 1685 | -7.1 | 687 | 1.9 |
| 1100 | 1574 | -8.3 | 600 | 2.1 |
| 1200 | 1466 | -9.7 | 520 | 2.4 |
| 1300 | 1362 | -11.2 | 449 | 2.7 |
| 1400 | 1263 | -12.9 | 386 | 3.0 |
| 1500 | 1168 | -14.7 | 330 | 3.4 |
Muzzle velocity 3000 fps is the factory figure from a 26 in test barrel. Expect your own barrel to read a little differently. Velocity is color coded green supersonic, yellow transonic, red subsonic; treat transonic and subsonic rows as approximate.
Trajectory
FAQ
What barrel length and twist should I run?
A 1:7.5 twist for the heavy 105 to 115 grain 6mm match bullets the cartridge is built around, and a 26 inch barrel to collect its velocity. A lighter-bullet varmint build could use a slower twist, but that is not what the 6-284 is for; smaller 6mm cases do that job with less powder and far longer barrel life.
Is the 6-284 Winchester a SAAMI cartridge?
No. The 6-284 Winchester is a wildcat with no SAAMI or C.I.P. standard. You neck down factory .284 Winchester brass to 6mm and handload it; there is no factory 6-284 ammunition. The dimensions on this page are derived from the parent .284 Winchester and are approximate.
How is barrel life?
Short, by design. The 6-284 is a thoroughly overbore 6mm, a large case driving a small bore, so it erodes the throat quickly. Expect match accuracy to fall off somewhere in the low thousands of rounds, sooner than a 6mm Creedmoor and far sooner than a .243 Winchester. Modern shooters reach for the more efficient 6mm cases because of it.
How does it compare to the 6mm Creedmoor and 6 Dasher?
The 6-284 has more case capacity and a little more velocity potential at the top end. The 6mm Creedmoor and 6 Dasher are more efficient, easier on barrels, and either standardized or widely supported, so they reach nearly the same place for less powder and longer barrel life. The 6-284 is a heritage choice for shooters who want maximum 6mm speed from .284 brass and accept the cost.