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Do rifle's perform better when barrels are longer? In other words, is there a correlation between barrel length and bullet performance?
We've all heard it before: "Velocity will decrease _____ (fill in the blank) feet per second for every inch shorter the barrel." Matched by the inverse, that for every inch of barrel length you will increase bullet velocity by X-feet per second. In fact, after acknowledging that exact velocities can't be calculated, Hornady says this on their website: "Velocity gains or losses will be approximately 25 to 35 feet per second per inch of barrel difference."
How true is this really? The correlation between barrel length and velocity obviously has its limitations due to physics. The conversion of powder into gas will only push a bullet so far before friction between the bullet and the lands inside a barrel cause a diminution in velocity. And the reverse, make a barrel ever shorter and bullet velocity will fail due to waste of energy out the muzzle instead of behind the bullet.
Clearly, there must be an optimal barrel length at which the energy provided by powder maximizes bullet velocity at the moment it exits the muzzle. But can that optimal barrel length be predicted? The problem with attempting to calculate optimal barrel length is many-fold. First, as alluded to by Hornady, every rifle seems to behave differently, even the same model rifle in the same caliber from the same manufacturer. Further complicating the issue is the extraordinary impact of different powders and their burn rates used in the same caliber, the different bullets and bullet materials used, different barrel twist rates, different barrel materials, ad infinitum. Putting aside muzzle velocity, terminal velocity can also be impacted by factors like bullet tip (acetal and delrin polymers are known to deform starting at 150 degrees Fahrenheit, and heat transfer will be higher with barrel length friction). To accurately establish optimal barrel length is nigh impossible.
While exactness for a given rifle may not be attainable, perhaps we may nonetheless achieve a general "rule of thumb" for optimal barrel length. For an indicator of what that optimal barrel length may be we might, for example, look at published muzzle velocities provided by ammunition manufacturers. It is probably fair to assume three things in that regard: First, manufacturers are attempting to publish maximum velocities in order to better market their ammunition. Second, manufacturers are publishing numbers borne of real-world (Doppler radar) velocities from a real-world rifle, not just a computer calculation. And third, ammunition manufacturers will use a selected optimal barrel length depending on the cartridge in order to maximize velocities (not to mention different powders for variabled burn rates). I wish I could tell you how each ammunition manufacturer does all that, but I can't and, further, it may be considered non-disclosable proprietary information.
Given all the above variables and limitations, the only way to reasonably calculate optimal barrel length (as an estimate, not an exact result) is through math. Fortunately, Doctors Pirooz Mohazzabi and Brad Shefchik from the Department of Physics, University of Wisconsin-Parkside, Kenosha, have created such a formula based on the principles of thermodynamics. Critically, they tested their formula against real-world experimental data and found that it is a highly accurate predictor of optimal barrel length. For those who want to know, here is a link to their report as a PDF file:
http://www.forum.thegunman-bg.com/download/file.php?id=27308
We've all heard it before: "Velocity will decrease _____ (fill in the blank) feet per second for every inch shorter the barrel." Matched by the inverse, that for every inch of barrel length you will increase bullet velocity by X-feet per second. In fact, after acknowledging that exact velocities can't be calculated, Hornady says this on their website: "Velocity gains or losses will be approximately 25 to 35 feet per second per inch of barrel difference."
How true is this really? The correlation between barrel length and velocity obviously has its limitations due to physics. The conversion of powder into gas will only push a bullet so far before friction between the bullet and the lands inside a barrel cause a diminution in velocity. And the reverse, make a barrel ever shorter and bullet velocity will fail due to waste of energy out the muzzle instead of behind the bullet.
Clearly, there must be an optimal barrel length at which the energy provided by powder maximizes bullet velocity at the moment it exits the muzzle. But can that optimal barrel length be predicted? The problem with attempting to calculate optimal barrel length is many-fold. First, as alluded to by Hornady, every rifle seems to behave differently, even the same model rifle in the same caliber from the same manufacturer. Further complicating the issue is the extraordinary impact of different powders and their burn rates used in the same caliber, the different bullets and bullet materials used, different barrel twist rates, different barrel materials, ad infinitum. Putting aside muzzle velocity, terminal velocity can also be impacted by factors like bullet tip (acetal and delrin polymers are known to deform starting at 150 degrees Fahrenheit, and heat transfer will be higher with barrel length friction). To accurately establish optimal barrel length is nigh impossible.
While exactness for a given rifle may not be attainable, perhaps we may nonetheless achieve a general "rule of thumb" for optimal barrel length. For an indicator of what that optimal barrel length may be we might, for example, look at published muzzle velocities provided by ammunition manufacturers. It is probably fair to assume three things in that regard: First, manufacturers are attempting to publish maximum velocities in order to better market their ammunition. Second, manufacturers are publishing numbers borne of real-world (Doppler radar) velocities from a real-world rifle, not just a computer calculation. And third, ammunition manufacturers will use a selected optimal barrel length depending on the cartridge in order to maximize velocities (not to mention different powders for variabled burn rates). I wish I could tell you how each ammunition manufacturer does all that, but I can't and, further, it may be considered non-disclosable proprietary information.
Given all the above variables and limitations, the only way to reasonably calculate optimal barrel length (as an estimate, not an exact result) is through math. Fortunately, Doctors Pirooz Mohazzabi and Brad Shefchik from the Department of Physics, University of Wisconsin-Parkside, Kenosha, have created such a formula based on the principles of thermodynamics. Critically, they tested their formula against real-world experimental data and found that it is a highly accurate predictor of optimal barrel length. For those who want to know, here is a link to their report as a PDF file:
http://www.forum.thegunman-bg.com/download/file.php?id=27308
