|180 gr. Model 21|
|Recommended Twist||10 inches/turn|
|180 gr. Model 21|
|Twist at Sg = 1.0||13.2 inches/turn|
|Twist at Sg = 1.5||10.8 inches/turn|
|Jump Sensitivity at Sg 1.0||.114|
|Jump Sensitivity at Sg 1.5||.140|
|Custom Drag Model (.drg format)||Download View|
|QuickLOAD Bullet File (.bul format)||Download View|
Data assumes a standard sea level atmosphere - 59°F, 29.92 mmHg, 0% relative humidity
The label "VLD" is not one that has a specific meaning, and we don't use it. We won't be offended if you call them VLDs, however, since the M21 does have an aggressive secant ogive that many would consider to be VLD-like. That said, we have not found the M21 to be any more difficult to tune than other match grade bullets. We do suggest trying a large range of seating depths, however. Every rifle is different, so be sure to test thoroughly. You'll be rewarded with some stellar groups and a BC that's competitive with the best in its class.
No. Trimming and or pointing the meplat of the Model 21 may increase their BC slightly if done with care and the proper knowledge. It is also risky, and can result in damage to the core-jacket interface or undesirable deformation if done improperly. We have found that most competitors interested in pointing prefer to do it themselves. We recommend caution - don't go pointing a whole box of bullets until you're sure that your process is actually generating the results you want. It's not worth ruining a box of expensive carefully crafted bullets, or worse yet, ruining your score at a match. I don't point my personal bullets - I use them straight out of the box.
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. The BCs are calculated and sanity-checked by range testing. 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 best matches the custom drag curve to a standard drag curve (G1 or G7). 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.
Any 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 dispersion due to unbalance or tipping. The higher the jump sensitivity, the less accurate a bullet will be, all else equal. Jump sensitivity is dependent on twist - 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, and it helps to illustrate the accuracy loss due to faster than necessary twists. The jump sensitivity factors we report assume a muzzle velocity of Mach 2.5.
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.
In our detailed, 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 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 twists are calculated for standard atmospheric conditions at sea level. In most cases, this is a conservative estimate and you can effectively use a slightly slower twist rate.
The Lapua Edition of QuickTARGET Unlimited specifies a file format for custom drag models, which is what we have chosen to use. Unfortunately, the software is a bit out of date, but you can still use it on older systems. We've published the files anyhow, as our hope is 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. Our .bul file includes G7 BCs only.
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. Be safe and follow standard reloading practices. If you are new to reloading get help from an experienced reloader.