Named after its 21 caliber ogive, the Model 21 (M21) was created to balance low drag, high precision, and repeatable stability in a match-grade hollow point boattail design. M21 is currently available in .308 caliber with weights of 185 and 200 grains.
|185 gr. Model 21||200 gr. Model 21|
|Weight||185 gr.||200 gr.|
|Minimum Twist (Sg = 1.0)||XX inches/turn||XX inches/turn|
|Recommended Twist (Sg = 1.5)||XX inches/turn||XX inches/turn|
|Custom Drag Model (.drg format)||Download||Download|
|QuickLOAD Bullet File (.bul format)||Download||Download|
|Buy Now||Buy Now|
Data assumes standard sea level atmosphere - 59°F, 29.92 mmHg, 0% relative humidity
We know how annoying it is to find that BCs have been inflated by manufacturers. Although the marketing copy uses an aspirational "we", Bison Ballistics is a one-man shop. I don't have a Doppler radar or a fully-featured ballistics laboratory. I do have access to 1,900 yard range for testing and some sophisticated engineering software. Initially, the BCs will be calculated and sanity checked by range testing. At some point I will be getting the bullets' drag measured by radar. Any BC published here is and will always be the best objectively determined estimate available given the means I have at my disposal. If I find any of them to be inaccurate, they will be adjusted.
We calculate the drag coefficient as a function of Mach number, and then determine a BC that matches a standard drag function at Mach 2.5. We find our bullets are a very close match to the G7 drag function during supersonic flight. There is typically a divergence at transonic and subsonic speeds, where it is advisable to use a custom drag model.
An time a standard atmosphere is needed, we use the ICAO (International Civil Aviation Organization) standard.
Jump sensitivity is a number that tells us how sensitive a bullet is to unbalance or tipping. The higher the jump sensitivity, the less accurate a bullet will be, all else equal. Jump sensitivity is dependent on bullet geometry and twist rate - a higher twist will increase jump sensitivity. We provide jump sensitivity factors so that you can make relative comparisons between bullets to get an idea as to the balance between ballistic performance and accuracy potential. A higher jump sensitivity bullet will not always shoot bigger groups in any given rifle, but we find it helpful as a rule of thumb.
Our bullets are a better fit for G7 BCs. We provide G1 BCs only as a convenience so that you can compare our bullets with those from other manufacturers who use G1 BCs. For actual ballistic calculations, use a G7 BC or our custom drag models if your software supports them.
On our detailed spec sheets, you'll find two minimum twist numbers. The first refers to a gyroscopic stability factor (Sg) of 1.0, which is the absolute theoretical minimum required for stable flight. The second refers to an Sg of 1.5. Based on the research done by Applied Ballistics, 1.5 is the stability factor required to minimize yaw and drag. We recommend using a rifling twist that will give you an Sg of 1.5 for long-range shooting. The recommended twits are calculated for standard atmospheric conditions at sea level. In most cases, this is conservative.
The Lapua Edition of QuickTARGET specifies a file format for custom drag models, which is what we have chosen to use. It is our hope that more ballistics calculators will adopt it or another standard format for custom drag models. If your ballistics calculator is able to use custom drag models, but requires another format, the .drg is pretty easy to convert - it's just a text file with a list of Mach numbers and drag coefficients.
A .bul file is is a bullet data file that can be imported into the QuickLOAD internal ballistics program. It contains bullet geometry, weight, and other information needed by QuickLOAD. See the QuickLOAD user manual for instructions on how to install a .bul file and for details on the format.
We do not have tested loading data for our bullets. We recommend that you invest in QuickLOAD and download our free .bul files. As always, and especially when using nonstandard overall lengths or wildcat cartridges, start 10% below the maximum charge weight and work up, looking carefully for signs of pressure along the way. QuickLOAD is good, but it's just software. When theory and reality collide, reality always wins. Be safe.