MUZZLE VELOCITY IS NOT A CONSTANT

Why Long Range Shooters Need to Periodically Chronograph their Rifles and Ammunition

By Mark V. Lonsdale

One of the most important data points that needs to be entered into any ballistic program or solver, such as AB Analytics, is the muzzle velocity (MV) of the selected rifle and ammunition combination. This is in conjunction with caliber, ballistic coefficient (BC), bullet length, barrel twist rate, and scope height above bore. But muzzle velocity is not something a shooter should measure just once and then assume it is always correct, even though many have made this mistake. Or even worse, they use the published muzzle velocity off the box of factory ammunition. While MV is less important for plinking or hunting under 300 yards, it is critically important for the long range shooters (600-1500 yards) and extreme long range shooters (1500+ yards).

20180128_141936_resized

Chronograph with the Labradar 

There are a number of reasons handloaded ammunition can change in muzzle velocity, to include, increasing the powder charge by as little as 0.1 grains, changing type or weight of projectiles, changing primer brand, and changing neck tension or crimp on the bullet. Any changes to the loading components or procedure demands a trip to the range to chronograph the ammo.

Factory ammunition can also exhibit wide changes in muzzle velocities. First, when you purchase different lot numbers of factory ammo, there will usually be a change in muzzle velocity. This is often more noticeable with plinking or hunting ammo and less with match grade ammo, but even match grade ammo could have a shift of 10-40 fps.

Between lot numbers, the factory may have changed to a different powder, a different primer, or a different lot number of projectiles. Then there are environmental effects. Changes in the season, ambient temperature, humidity, barometric pressure, and elevation above sea level will all affect muzzle velocity and the resultant point of impact (POI). This will be proportionate to the extent of the swing in temperature, gain in elevation, or drop in barometric pressure.

A6 Medium Kestrel

6.5 Creedmoor used for testing. Atlas Tactical action, 24″ Bartlein 1:8″ barrel in a McMillan A6 stock and topped with the Leuopld Mark 5HD 5-25x scope   

Additional changes in muzzle velocity will occur as a new barrel breaks in, wears, or if the shooter adds a muzzle brake or suppressor.  As an example, the following MVs were pulled from one of the 6.5 Creedmoor rifles I am testing. All the test ammunition is factory ammunition from the same lot numbers. The first column was from a new 24” Bartlein barrel with a 1:8” twist.

Factory Ammo                            Jan 2018            July 2018                         August 2018

                                                          New barrel      added muzzle brake    200 rounds

Federal Berger 130 Hybrid      2,830 fps           2,866 fps                         2,892 fps

Hornady Match 140 ELD          2,726 fps           2,746 fps                         2,780 fps

Hornady Match 147 ELD          2,670 fps           2,721 fps                         not tested

The above example shows a  51 – 62 fps increase in velocity from a new barrel, to 100 rounds and the addition of a Piercision muzzle brake, and then at 200 rounds with the brake. If I was still using the original MV, this would result in a significant error in predictive POI at 1,000+ yards.

6.5CM Ammo

Factory ammunition used in 6.5 CM testing

Whichever ammunition a shooter selects, to get accurate ballistic data or firing solution, the shooter should chronograph the ammunition periodically to track wear or changes in the barrel; if there is a big shift in ambient temperature, as in 85 F summer to 32 F winter months; or a significant shift in elevation, as in sea level to 6,000+ feet. Keep in mind that 6,000’ on a hot day could result in a density altitude of 9,000+ feet, as we found this summer during the King of 2 Miles at the NRA Whittington Center in Raton, New Mexico.

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The second 6.5 Creedmoor being used for testing. Rem 700 action, 24″ Krieger Heavy Palma barrel, 1:8″ twist; McMillan A6 stock; Leupold Mark 4 6.5-20x scope 

Finally, the shooter should know that not all chronographs are equal. A cheap chronograph that may have a 40-50 fps error from actual MV, will result in a significant error in data input and all resultant firing solutions. This goes back to the computer adage of, “garbage in, garbage out” and that a computer is only as good as the data inputted.

Chrono 10.15.17

Early two screen type chronographs are sensitive to light condition and placement. 

Conclusion – accuracy, diligence, and consistency is critical in all aspects of reloading and long range precision shooting, and an accurate MV is a critical part of that process.

END

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About Mark V

Dedicated shooter, seeker, traveler, teacher, trainer, educator
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