Effects of air Temperature on Subsonic load speed

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ropedrag
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Effects of air Temperature on Subsonic load speed

Post by ropedrag » Tue Dec 24, 2019 4:50 pm

As I'm working up different subsonic loads for bulk reloading I'm beginning to wonder how much of a speed increase may occur if I develop/test a load in 40 degree weather and then shoot it on a 100 degree day. I've found an accuracy and speed node but it's not too far from going supersonic (50 fps +/-). The last thing I want to do is load up a 1000 rounds that will be supersonic in the summer!!!

All other ballistics aside apart from being subsonic below the speed of sound.

Or stated a different way....

What I'm trying to learn is, while using my powder(s) Accurate 1680, Hogden CFE BLK, and VV N120, will the speed change (increase) by shooting a cartridge that's 40 degrees vs one that's 100 degrees equal more than the FPS increase in the speed of sound over the same temperature increase.

I've learned, (somewhat contrary to what a lot of people run with), that the speed of sound is the same on a mountain top as it is at sea level if the air temperature is the same. The density of the air has no effect, because the ratio of the air pressure to the density is always constant. So for our extremes shooting purposes we'd likely operate within the following range.... speed of sound on a -40 degree day = 1004, and on a 125 degree day its = 1185. A delta increase of 181 fps. So what powders would be equal to or less than this delta over these temperatures?

Its a bit crazy finding information but on the Accurate website they do state "Most of our powders are not insensitive, and will show some effect at hot and cold temperatures. However, we test at -40F and +125F and the deviation in most cases are ca 3% to 5% at these extreme levels. Therefore most shooters do not notice much difference under normal practical hunting conditions." 5% of a 1000fps load over this temp range is only 50 fps vs 181 fps for the speed of sound range over the same temp range so basically, not an issue to worry about.

On Hogdon's site referencing "extreme powders" they compare a limited number of powders in different cal's and loads to theirs. And while these FPS differences would effect accuracy/distance it appears only one powder of their limited list would make a difference in the potential to go supersonic shot on a hot day if loaded to be on the edge of subsonic on a cold day. It also stands to reason that they found the most extreme temp sensitive powders for comparison as it makes their powder look better.

So, Accurate's site would indicate, don't worry about it while Hogdon's would crack the door to concern with some (very limited?) powders.

That said it appears from my limited research that I wont have issues with regard to staying below the speed of sound developing loads in the winter and shooting them in the summer. The speed increase of the round brought on by higher temperatures typically lags the increase of the speed of sound brought on by the same higher temperatures.

My load of Accurate 1680 that crono's at 1,044 on a 40 degree day should not go supersonic at 1159 on a 100 degree day.

Again, all other ballistics aside apart from being subsonic below the speed of sound.

Agree? It makes sense.
Disagree? I've bumped my head.
Thoughts? To what speed do you park your load development, why

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Re: Effects of air Temperature on Subsonic load speed

Post by TRshootem » Tue Dec 24, 2019 5:49 pm

I try to have my heavy subs around the 900 fps spot so I'm good at cool temps around the upper 30's. For the most part this has worked out, seldom am I going to be shooting subs at -20*. I do have some loads that are 7-800 fps but those are a bit slower in the 10.5" AR vs the 16" bolt rifle. Here on the East slope of the Continental Divide, weather, wind, and temps can swing fast and furious. If the subs crack when out playing with toys...meh. I would add that accuracy dictates the settled velocity, depending on the bullet weight etc.

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Re: Effects of air Temperature on Subsonic load speed

Post by Blowout » Wed Dec 25, 2019 6:32 pm

ropedrag wrote:
Tue Dec 24, 2019 4:50 pm

I've learned, (somewhat contrary to what a lot of people run with), that the speed of sound is the same on a mountain top as it is at sea level if the air temperature is the same. The density of the air has no effect, because the ratio of the air pressure to the density is always constant. So for our extremes shooting purposes we'd likely operate within the following range.... speed of sound on a -40 degree day = 1004, and on a 125 degree day its = 1185. A delta increase of 181 fps. So what powders would be equal to or less than this delta over these temperatures...
Altitude change will change the velocity of the round. With increased elevation there is lower air density which will allow for an increase the velocity of the bullet everything else being equal. Altitude can’t be ignored as a contributing variable to a subsonic load. Similar effect as increasing temperature...
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Re: Effects of air Temperature on Subsonic load speed

Post by ropedrag » Thu Dec 26, 2019 2:22 am

Blowout wrote:
Wed Dec 25, 2019 6:32 pm


Altitude change will change the velocity of the round. With increased elevation there is lower air density which will allow for an increase the velocity of the bullet everything else being equal. Altitude can’t be ignored as a contributing variable to a subsonic load. Similar effect as increasing temperature...
Got it, from my research air pressure and density will not effect the speed of sound like temperature will, however the temperature will have some small effect on the powder and as you point out the air pressure and density will have an effect on the bullet.

This all has me thinking that a muzzle velocity in the low 900's is the best place to park a developing subsonic load. Added benefit as I've developing with my 9" barrel so if the need ever arises the round has a better chance to remain sub in a 16 barrel.

That said I just noticed that the Hornady 300 BLK "Sub-X" 190 gr has a muzzle velocity of 1050 with a 16" barrel.

After some more though I'm going to pick my more accurate load between my 2 finalists (1026 fps vs 1044 fps) of the Hornady brass, accurate 1680 w/Sierra Match King 200gr HPBT for 500 rounds and use the other 2 loads under development for the slower more conservative bulk loading.

Thanks for the comments, I've so much to learn its not even funny :lol:

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Re: Effects of air Temperature on Subsonic load speed

Post by dellet » Thu Dec 26, 2019 9:55 am

Blowout wrote:
Wed Dec 25, 2019 6:32 pm
ropedrag wrote:
Tue Dec 24, 2019 4:50 pm

I've learned, (somewhat contrary to what a lot of people run with), that the speed of sound is the same on a mountain top as it is at sea level if the air temperature is the same. The density of the air has no effect, because the ratio of the air pressure to the density is always constant. So for our extremes shooting purposes we'd likely operate within the following range.... speed of sound on a -40 degree day = 1004, and on a 125 degree day its = 1185. A delta increase of 181 fps. So what powders would be equal to or less than this delta over these temperatures...
Altitude change will change the velocity of the round. With increased elevation there is lower air density which will allow for an increase the velocity of the bullet everything else being equal. Altitude can’t be ignored as a contributing variable to a subsonic load. Similar effect as increasing temperature...
How much will my muzzle velocity change per 1000 foot change in elevation?
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Re: Effects of air Temperature on Subsonic load speed

Post by dellet » Thu Dec 26, 2019 10:27 am

If you develop a load in the coldest temps you shoot, winter, it will never go super in the hottest temps.

For the most part velocity increase due to powder/cartridge temp will be less than the increase of the sound barrier temp. Unless you leave your ammo in direct sun, or on the dashboard under glass, it just won’t happen.

The biggest problem is powders that become unstable at low temps and pick up speed in the cold rather than drop. Or developing your load at summer temps and shoot in the winter. The speed of sound/muzzle velocity relationship is the same. The sound barrier temp drops faster than muzzle velocity. If the load is borderline in summer, it will crack in winter.

If someone knows of a magic bullet that increases velocity, after it has left the barrel, I’m interested in trying some.

The key to subsonic shooting is interior Ballistics, the speed it leaves the barrel.
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Re: Effects of air Temperature on Subsonic load speed

Post by rebel » Thu Dec 26, 2019 5:31 pm

dellet wrote:
Thu Dec 26, 2019 10:27 am
If you develop a load in the coldest temps you shoot, winter, it will never go super in the hottest temps.

For the most part velocity increase due to powder/cartridge temp will be less than the increase of the sound barrier temp. Unless you leave your ammo in direct sun, or on the dashboard under glass, it just won’t happen.

The biggest problem is powders that become unstable at low temps and pick up speed in the cold rather than drop. Or developing your load at summer temps and shoot in the winter. The speed of sound/muzzle velocity relationship is the same. The sound barrier temp drops faster than muzzle velocity. If the load is borderline in summer, it will crack in winter.

If someone knows of a magic bullet that increases velocity, after it has left the barrel, I’m interested in trying some.

The key to subsonic shooting is interior Ballistics, the speed it leaves the barrel.
I cannot explain enough how right dellet's explanation is.
I can only follow that up with a Wiki explanation -

The speed of sound is the distance travelled per unit time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 metres per second (1,235 km/h; 1,125 ft/s; 767 mph; 667 kn), or a kilometre in 2.9 s or a mile in 4.7 s. It depends strongly on temperature, but also varies by several metres per second, depending on which gases exist in the medium through which a soundwave is propagating.

The speed of sound in an ideal gas depends only on its temperature and composition. The speed has a weak dependence on frequency and pressure in ordinary air, deviating slightly from ideal behavior.

In common everyday speech, speed of sound refers to the speed of sound waves in air. However, the speed of sound varies from substance to substance: sound travels most slowly in gases; it travels faster in liquids; and faster still in solids. For example, (as noted above), sound travels at 343 m/s in air; it travels at 1,481 m/s in water (almost 4.3 times as fast as in air); and at 5,120 m/s in iron (almost 15 times as fast as in air). In an exceptionally stiff material such as diamond, sound travels at 12,000 metres per second (39,000 ft/s)[1]—about 35 times as fast as in air—which is around the maximum speed that sound will travel under normal conditions.

Sound waves in solids are composed of compression waves (just as in gases and liquids), and a different type of sound wave called a shear wave, which occurs only in solids. Shear waves in solids usually travel at different speeds, as exhibited in seismology. The speed of compression waves in solids is determined by the medium's compressibility, shear modulus and density. The speed of shear waves is determined only by the solid material's shear modulus and density.

In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure for the speed of an object moving through the medium. The ratio of the speed of an object to the speed of sound in the fluid is called the object's Mach number. Objects moving at speeds greater than Mach1 are said to be traveling at supersonic speeds.
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Re: Effects of air Temperature on Subsonic load speed

Post by Blowout » Thu Dec 26, 2019 6:49 pm

Thanks for the correction. So BC is increased with altitude because of lower density but velocity is constant.
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Re: Effects of air Temperature on Subsonic load speed

Post by dellet » Thu Dec 26, 2019 7:58 pm

Blowout wrote:
Thu Dec 26, 2019 6:49 pm
Thanks for the correction. So BC is increased with altitude because of lower density but velocity is constant.
The bullet's flight is different, BC, but will basically have the same starting muzzle velocity. So example numbers, A muzzle velocity at 100 feet altitude of 1000fps, will still be 1000 fps at 10,000 feet, as long as the barrel and cartridge are the same temperature for both shots. The bullet drop or point of impact at 100 yards will change according to air conditions. One bullet will lose speed faster do to air conditions/resistance.

This can and does happen in the mountains all the time and people die from it. The California coastal ranges are famous for it. You have 10,000 foot peaks with coastal forests on one side and desert on the other. At 7000 feet on the desert side the temp is 90 degrees. Coastal side of the same mountain, less than one mile west on the map at the same altitude, temp is 30 degrees. Altitude is not what changes the conditions.

Different powders behave differently to temp changes. Speed of sound is predictable with temp. It basically changes 1 degree/1 fps. If your powder changes muzzle velocity faster/slower than the temp changes the velocity required to stay subsonic, you can get a crack. That can be at both ends of the scale in the right conditions.
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Re: Effects of air Temperature on Subsonic load speed

Post by popper » Thu Dec 26, 2019 8:03 pm


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