How To Chasing Muzzle Velocity Standard Deviation and Extreme Spread for Precision Rifle Shooting Sean Murphy August 15, 2019 Join the Conversation For a long-range shooter, understanding your rifle’s performance is key for connecting with targets. Science and technology have provided easy access to ballistic computers and information, but computers cannot always account for real-world variations. While we enjoy rifles, optics, and ammunition of extremely high quality, the average muzzle velocity used to calculate ballistics does not account for velocity variances between shots. We can measure this and statistically analyze data for probability, but it’s up to a shooter to understand their system and these variances to make hits on target. 5 shots recorded with a MagnetoSpeed v3 chronograph A rifle and ammunition combination that yields great accuracy has always been prized by shooters. An often-overlooked quality is the consistency of velocity for the same combination. Accuracy can be measured in group size and a ruler, but velocity and its consistency are measured with a chronograph. Firing your chosen ammunition over a chronograph provides the essential muzzle velocity for building an accurate ballistic profile. This chronograph data also allows analysis of the ammunition via several statistics including the velocity average, standard deviation, and extreme spread. At close to mid-range these variances are usually not significant enough to worry about, but as distance increases velocity variances could create a miss if they were large enough. Definitions The average is the calculated central number of a sample. This is the number placed into the ballistic computer as your muzzle velocity. The standard deviation (aka “SD”) is a measurement that shows how much variation from the average number exists in a sample. A low number indicates most numbers are close to the average. For purposes of ammunition, the lower the SD the better the ammunition. The extreme spread (aka “ES”) is the spread between the highest and lowest velocity in a group. Sample Size Even if shooters do evaluate their ammunition it might not always be at a statistically significant number for long-range shooting. Without getting too deep in valid statistical analysis, for testing velocity consistency more rounds fired is better. Just like with shooting groups for accuracy, 3-shots and even 5-shots do not always tell the full story. While small sample sizes can be an indicator of an average performance, at least ten shots can give us a decent understanding of velocity variances. A really good check for performance would be to test around parameters of which the rifle will be used. For example, a PRS shooter might test 10-12 rounds in 90 seconds to 2 minutes as a stage would be shot, or a Benchrest, F-class or other target shooter would record a normal string of fire under match conditions. 10 shots recorded; SD of 10.0 fps and an ES of 35 fps Relevancy So why does this matter? When we are shooting targets at long distance we need ammunition to be consistent. If a load has a high extreme spread/standard deviation, our probability of hitting the target is reduced because the bullet could miss high or low simply due to the velocity being different with each shot. The effects of this are relative to cartridge, the target size, and target distance. It is important to determine the level of precision needed from a rifle system in order to determine the amount of effort or expense dedicated to precision ammunition. A 5.56 carbine shooting at large targets inside of 300 yards will not see enough variation to create misses from velocity changes; whereas a precision rifle trying to hit small targets at 1000 yards will need more consistent ammunition. To help understand this, we will look at several extreme spread brackets relative to a 1 MOA target. A 1 MOA target standard is generally considered a precision shot regardless of discipline or application. The graphic illustration ONLY highlights the potential of velocity variation, not including environmental effects, inherent mechanical accuracy of the rifle system or shooter influenced errors. Cartridge Examples: 5.56mm/.223 – 77gr Sierra MatchKing at 2700 feet per second (fps) .308 Win – 175gr Sierra MatchKing at 2600 fps 6.5 Creedmoor – 140gr Hornady ELD-M at 2750 fps .338 Lapua – 300gr Berger Hybrid at 2750 fps Velocity Extreme Spread Brackets: ES bracket of 20fps (+/- 10fps) – All shots would be within 10 feet per second above or below the average, having an extreme spread of 20. Having ammunition which this performance or better is generally considered ideal for most uses. ES bracket of 40fps (+/- 20fps) – All shots would be within 20 feet per second above or below the average, having an extreme spread of 40. ES bracket of 60fps (+/- 30fps) – All shots would be within 30 feet per second above or below the average, having an extreme spread of 60. Ammunition with this performance or worse could have a hard time hitting small targets at long-range, even if it groups well. As the illustration shows, at 250 yards even a significant change in velocity is going to have an insignificant change on the ballistic path regardless of the cartridge. At 500 yards, the less ballistically efficient 5.56 and .308 cartridges will require most of the 1 MOA target size just for velocity variance alone if there is a high ES and SD. The more efficient 6.5 Creedmoor and .338 Lapua would still hit the small target even with a higher ES/SD. At 1000 yards the 5.56 and .308 cartridges would be tough shots anyways due to running out of velocity, but even with a low ES the velocity variation would cover most of the target. The 6.5 Creedmoor and .338 Lapua would still require a tight ES to get a good hit on the target but because they are more efficient some velocity variation can still be tolerated. All other factors aside, even with a tight ES at 1500 yards the 5.56, .308 and 6.5 Creedmoor variations would be greater than the target. Even the .338 Lapua with its highly efficient bullet would still require good ammunition to connect on the small target. Extreme Spread and Standard Deviation Together When evaluating ammunition, it is important to evaluate both the standard deviation and extreme spread. The examples below show how loads can have an identical extreme spread, but different standard deviations. It is important to understand what your load is doing as it will increase your hit probability through consistency. This is especially true as distance increases, as we tend to correct for misses based upon the last shot’s information. If we know our ammunition consistency and if we have velocity variances similar to or larger than the target, it would be important to consider not making minor elevation corrections to keep the majority of shots on the target instead of chasing errant shots created by velocity differences. The graphics below show an Extreme Spread of 20 fps, but one has standard deviations that vary between a high and low average (harder to predict result), while the other is more central with a few outliers (more representative of the average). These are just two examples of many of a population that shows it’s important to test your ammunition and understand how your rifle performs with both ES and SD metrics. Reality Check It is important to understand your rifle’s performance, especially when shooting across long distances. However, applying how much it matters and determining if spending the time and money chasing more consistent ammunition is also important. Often with precision rifles, we tend to chase better groups or very low ES/SD numbers when the group sizes or velocity performance are good enough as we expect perfection from the system. A reality check on do you need to keep chasing your ammunition or just get to shooting is easy…start with considering some of the longer distances the rifle might be used at. Enter the average muzzle velocity into your ballistic calculator and get your bullet drop data for the farther expected distances. Now enter the highest and lowest ES numbers recorded and see how much the drop data changes (if any). You might find that at even long-range, decent factory ammunition or average hand loads will be consistent enough for connecting with targets. 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