Temp. Effect on Velocity


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Ak Guy
March 18, 2008, 01:33 AM
As the air temp (and cartridge temp) increase, is it safe to assume that the velocity will increase? Are there any rules of thumb to guess by how much?
i.e. say I chrono a 9mm round at 1100fps at 20 deg F, what's it likely to be at 80 deg F ??

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Drail
March 18, 2008, 07:13 AM
Actually it can go either way depending on the powder type. I have seen IPSC shooters in Arizona stash their loads in a cooler until right before they shoot because the velocity will drop enough to cause them not make Major power factor. Most powders show a pressure rise when they get to freezing temps.

BsChoy
March 18, 2008, 02:20 PM
Drail, I have read just the opposite. Temp increase causes the pressure to increase. Cold should lose FPS.

Halo
March 18, 2008, 02:31 PM
Temperature and pressure are directly proportional. One of the gas laws as discovered by French chemist Joseph Louis Gay-Lussac reads as "The pressure of a fixed amount of gas at fixed volume is directly proportional to its temperature in kelvins." Chemical reactions also tend to occur with more vigor in higher ambient temperatures. There are some notable exceptions to that, but combustion of gunpowder isn't one of them.

I believe somewhere in my Speer Manual #13, it even mentions that some maximum power loads developed for winter may be overpressure if fired in warmer conditions. I'll have to search for the exact statement when I get home.

Shoney
March 18, 2008, 05:11 PM
Halo is absolutely correct. PV=nrT

However, extreem cold is another story. I have hunted at 30 to 45 below F. on many occasions. The cold is a factors on other compnents of the rifle other than the powder in the cartridge. My experience showed that loads worked up at 20 degrees F. were showing pressure signs while the performance of the bullets indicated the velocity was much slower.

All of the game I have shot at those subzero F. temps showed that the bullets were indicating much slower velocity, Exampes: 6MMRem 100gr Nosler Partitions and 100 gr Hornady SPBT, which normaly blow thru deer would not penetrate completely thru. 300WinMag 200gr Nosler Partitons would not completely penetrate an elk when under normal conditions they barely slowed down. Similar results with 270 and 30-06.

My scientificly based conclusions are that at extreemly cold temps metal shrinks a little bit (scientific fact). I also believe that the friction coeficients of copper and steel are slighly altered. The combination of reduced diameter bore and altered friction produced the high pressure by slowing the bullets markedly.

I have not noticed any variation at or around zero.

Ol` Joe
March 18, 2008, 07:19 PM
The figure I`ve seen is about 1.5 fps per deg F, in as I remember, a older Lyman manual.
I have found more variation in velocity going from ~60* F up in temp then in colder temps with 270/708 loads. I`ve never paid much attention to handgun cartridge speeds once I`ve decided on a load.

Ak Guy
March 19, 2008, 01:23 AM
Thanx for the resposes. It seems like the consensus is that the vel will increase w/ temp increase. Good. It seems like it should, given that chemical reactions tend to be more volatile as temps increase.
I'm going to be shooting a couple matches in Phoenix / Las Vegas soon, but will have to chrono here in Alaska, at 20-40 deg F. As long as vel goes up w/ temp I'm fine...... I just don't want to flunk the power factor test !!

moosehunt
March 21, 2008, 03:59 PM
There are different areas to consider regarding temperature. Temperature of the powder, temperature of the barrel for examples. Temperature of the barrel will have greater effects on preasure than will temperature of the powder, though both will effect preasure significantly, hence velocity. Keeping loads cool in warm temperatures will help in holding preasure down, but that barrel is going to be warmer, and that will have a greater effect on raising preasure. Temperature of the primer seems to have little significant effect, though technically, warmer temperature will effect the primer, just not significantly. Air temperature also will effect velocity. Warmer air is less dense, hence produces less resistance to the bullet, thus velocity will go up compared to colder air. Temperature will also effect ductility of the metals. Unless extremes are involved, it is not real significant, but in warmer temperatures, the steel of the reciever will will expand more upon firing, resulting in a larger cumbustion chamber, hence lower preasure, the opposite if colder.

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