Range Report Does elevation affect muzzle velocity?

[surgeon260]

Looking for some input from the experts. Personally I have never observed a statistically significant difference in muzzle velocity in practice as a result of air pressure, and from a theoretical perspective I don't understand how or why it would make a difference. There are also many confounding variables such as temperature so drawing conclusions as to the root cause of a velocity difference is a tenuous exercise at best. Air pressure at sea level is less than 15 psi. Compared to 65,000 psi of chamber pressure, it seems like any velocity differences as a result of more or less air resistance in the barrel would be orders of magnitude too small to observe in terms of measured muzzle velocity.

What do you guys think?

 

[_Windrider_]

Temp. It’s all temp. The higher the temp the higher the MV.

 

[Tony1320]

The higher altitude you are the more muzzle velocity you retain down range. Air is less dense at 5000 ft DA than at sea level.

If you had doped 30 moa at 69f temperature at 5000 ft DA and went somewhere at sea level and 69f temperature you would need to add more elevation because at sea level your muzzle velocity drops faster over distance.

Here is a good article
https://www.snipershide.com/density-shooting/

Bryan Litz also talks in detail about this subject in his books

 

[_Windrider_]

The higher altitude you are the more muzzle velocity you retain down range. Air is less dense at 5000 ft DA than at sea level.

If you had doped 30 moa at 69f temperature at 5000 ft DA and went somewhere at sea level and 69f temperature you would need to add more elevation because at sea level your muzzle velocity drops faster over distance.

Here is a good article
https://www.snipershide.com/density-shooting/

Bryan Litz also talks in detail about this subject in his books

Yeah but the question was what affects velocity and the answer is temp. Density affects the rate of decay but has nothing to do with your velocity as the bullet leaves the muzzle.

 

[MarkCO]

Altitude is irrelevant. What matters is the mass per unit volume and temperature as factors which affect velocity decay enough to matter. That is why DA is a commonly used variable in ballistics, artillery, and aviation. The formula PV=nRT (Pressure x Volume = number of moles x Gas Constant x Temperature) is the ideal gas law and with much manipulation, you can come up with the formulas used in the solvers. Those formulas are not all the same and they have some of the assumptions made by the person writing the formulas to get as close to real world as they can. "Ideal Gas" is in itself an approximation. But it tells us that if T increases at constant volume and moles, Pressure must increase. If n decreases at constant volume and temperature, then P must decrease. Also, remember that there are interior ballistics (what happens IN the barrel) and exterior ballistics (what happens to the bullet in flight) and there is only a slight connection. In most cases, when Thermo guys start a calculation using some sort of a Monte Carlo system, we use one of the STP sets (for instance 59F and 14.7lb/ci) to get on the path and hand check the solvers we have programmed. There are others as Thermodynamics, Physics, Aviation guys always try to better it, but the current international standard is 0C and 1 bar.

So as we increase the grains of water in the air or n (just think of this as the number of molecules in a cubic foot of air) the resistance goes up because there is more "stuff" for the bullet to move out of the way (and why you see trace) so velocity decay increases. There is also some math about partial pressures, but that only has an effect of tenths of an inch at a mile. Form factor and how the bullet interacts at the nose and tail also affect velocity decay and they are "best" at one specific set of conditions.

Why you fly a plane (or a bullet) over water, or a much lighter or dark patch of earth, the DA is changed and that certainly affects the flight of the object. In some cases, you will shoot "short" and it others "long" as opposed to the data without those factors included.

 

[KenAZ]

Temp. It’s all temp. The higher the temp the higher the MV.

Is there a rule of thumb for the impact of the delta in temperature? If average MV is 2700 @ 85 degrees is there a way to calculate what MV would be at 40 degrees?

 

[_Windrider_]

Is there a rule of thumb for the impact of the delta in temperature? If average MV is 2700 @ 85 degrees is there a way to calculate what MV would be at 40 degrees?

You have to get your mv at that temperature to figure out the variation of muzzle velocity in feet per second per degree

Every ammo is different when it comes to temperature sensitivity so there’s no way to have a rule of thumb for that

 

[Ledzep]

To make that even more complicated, it's usually not a very linear increase. Typically from below freezing to 60-70 degrees is pretty low slope, then get grows quickly, especially over 100 degrees F.

Rules of thumb that will be sorta close; Hodgdon Extreme and temp stable RL powders will be ~0.2-0.6 fps per degree. Less stable extruded stuff is in the 0.7-2 usually. Spherical is typically 1.5-3.5. Lots of exceptions don't take any of that as gospel. Changing bullet weight or case volume can make the same powder produce differing temp stability.

Also, there's the effects (especially at increased temp) of changing moisture content in the powder, which is probably some of why there's faster increase the hotter you go.

 

[_Windrider_]

To make that even more complicated, it's usually not a very linear increase. Typically from below freezing to 60-70 degrees is pretty low slope, then get grows quickly, especially over 100 degrees F.

Rules of thumb that will be sorta close; Hodgdon Extreme and temp stable RL powders will be ~0.2-0.6 fps per degree. Less stable extruded stuff is in the 0.7-2 usually. Spherical is typically 1.5-3.5. Lots of exceptions don't take any of that as gospel. Changing bullet weight or case volume can make the same powder produce differing temp stability.

Also, there's the effects (especially at increased temp) of changing moisture content in the powder, which is probably some of why there's faster increase the hotter you go.

there is also the factor that OAT doesnt necessarily equal ammo temp. if you are on the range and keep your ammo out of the heat or cold while shooting, it could be 100 degrees outside but only 70 inside your bag or case or whatever. if you are out hunting, its probably safe to assume the ammo in the gun is damn close to OAT, but the reserve ammo may not be until its been exposed for a while.