Well this is my first post. I have been lurking for a long time but now need a little help.

I recently bought a chronograph and it has really opened up a whole new can of worms for me.

What have you all found the affect of outside temp is on velocity? I was shooting my .270 and .223 today in about 65F. What should I expent when it is 95?

Thanks for the help. Tons of great stuff here.

Paul

Welcome to the forum.

Your fps will be somewhat faster. Hodgdon Extreme powder is supposed to mitigate this effect.

It's really hard to tell how much effect there will be. The effect varies from cartridge to cartridge, and powder to powder.

I ran an experiment that separated chamber temperature and cartridge temperature. Chamber temperature was about 3X as important as ammunition temperature.

In that particular cartridge, with that particular Hodgdon Extreme powder, I got about 200 PSI per degree F of chamber temperature, and 70 PSI per degree F of ammunition temperature.

OTH, my 308 was absolutely dead temperature stable with Varget.

Welcome to Wonderland...

interesting, denton. how'd you separate chamber temperature from cartridge temp?

I've been thinking all along that the time a cartridge spends in the chamber matters as much or more than 20-80 degree ambient change.

For that reason, when chronographing, I put 5-10 rnds through before i start chronographing, and then i try to keep a cadence so that all my rounds spend the same time in the chamber.

i could be way off base there. i am at least attempting to remove that variable from my testing, but i have no idea how to separate it from cartridge temp.


btw, i was told by sierra that for double-base powders, they avg 2 fps per degree of temperature change. they said single-base change less.

taliv...

What you are doing is excellent practice.

I happen to have a little electronic thermometer, with a flexible thermocouple. I put a little dab of of grease on the barrel, just forward of the receiver, and tape a thermocouple to it.

Then, I do exactly as you describe--run the temperature up, pick an operating temperature, and every time the meter settles down to my selected number, pop off another shot.

Ambient temperature doesn't matter much.

The temperature of the cartridge matters, and the temperature of the chamber matters. If a cartridge sits in the chamber, only a 10 minutes are required for the powder to warm... stuck a thermocouple up through a primer pocket, and checked it once. Of course, one approach is to chamber your next round as soon as you fire... ammo and chamber temp become one and the same.

In my experiment I had warm ammo and cold ammo, and I did my shooting on two different days, where I could keep the chamber relatively cold, and then hot. There is a nifty little stat tool that lets you do multiple variables at once, with no increase in sample size, up to 5 variables.

As far as I can tell, there is an absolutely perfect way to separate chamber and ammo temperatures, but you can come close by shooting in the single-shot mode, and being very quick about it. If you can cycle the firearm in under 10 seconds, the change in temp of the powder is minimal.

In the next few weeks, I'll do another experiment to see what the effects of temperature are on variaous combinations of primer and powder. I suspect that primer temperature is actually the active variable, and I think I've figured out how to test that. We'll see what the data tell us.

Each question answered gives us at least two more questions. That is what makes this hobby interesting.

Denton wrote an interesting article on this subject. It may be found at this link.
http://www.shootingsoftware.com/ftp/Pressure%20Factors.pdf

cool. thanks for the info

are you sure you meant 10 minutes up there, denton? i can get a pretty severe burn touching the outside of my barrel in about a second and a half. seems like the powder would warm at least 20-30 degrees in the first 2-3 seconds in a hot chamber.

idaho, yep, that and making lots of noise. and smoke. oh, and blowing things up, of course.

Oh... I didn't mean temperature of the barrel. I meant that the temperature of the inner powder will be at whatever the chamber is, 10 minutes or so after you insert the cartridge.

.

I found little difference in small calibers. The point of impact of my 22-250 does not change with low or high temperatures whereas things change a lot with heavier bullets in bigger calibers. The point of impact of my 8x56R is about 20 cm lower with outside temperatures around -15 Celsius, a common temperature when hunting in Europe in winter. This goes for military surplus and handloaded ammo. So velocities seem to be a lot lower with cold ammo. When hunting you shoot a cold round from a cold weapon so the good target shooting practice to keep temperature at a given level does not apply. I never was able to measure velocities at low temperatures since my Chrony does not work when cold. Perhaps I should try to load a winter and a summer cartridge.

quote:

All temperature related.

All true, ALF, but mostly not my point.

My point is that ambient temperature probably has no effect in and of itself on internal ballistics. It's a radical statement, but if you think it through carefully, I think it has to be true.

Ambient temperature influences ammunition temperature, and ambient temperature influences barrel/chamber temperature, and those, not ambient temperature itself, are the primary actors in changing the pressure curve.

You can see my experiment, done in constant ambient temperature, with constant temperature ammunition, where pressure climbs linearly with chamber temperature.

Thought experiment: You go outside when it is just at the freezing point of water... nasty, cold drippy day. You have ammo heated to 40 C, and your rifle is heated to 40 C. If you put the ammo in the rifle, and fire it, how is it going to "know", and respond to, the outside temperature, until the bullet exits?

yeah, ALF, i don't think anyone's saying ambient doesn't (indirectly) affect IB. I'm just comparing the temperature of the chamber to the temperature outside relatively.


denton, i'm still not following you here. Let's say it's 50 degrees outside and after 10 shots or so, all the metal from the chamber to the end of the barrel is WAY over 100 degrees.

Are you saying if I take a cartridge that has been sitting outside, and place it in the chamber, that the brass (which conducts pretty dang well), and the as yet unfired powder will take 10 minutes to warm noticably? I just find that hard to believe.

I think i'll try to find some sort of thermometer and try that at the range next weekend. i.e. warm the gun up, insert cartridge, remove 30 seconds later and measure


on a related note, i doubt it matters much, but does brass expand much when heated? is it possible for it to heat up enough to affect neck tension?

Ask an old Vietnam sniper if the outside air temperature will make a differance when lining up on a target about 800 meters away. The hot temp of a sunny day will cause the charge to shoot with more FPS and cause the bullet to rise. Will make a good differance when popping a Cong head at that differance. After shooting several hunderd rounds each day you can adjust real good to changing temps. At night it is dead on most of the time.

taliv...

What I meant to say is that after about 10 minutes, it is safe to assume that the cartridge, and the entire powder charge, will be at the same temperature as the chamber (and even 10 minutes may be more than is required). Then all you have to worry about is one temperature, not two.

The powder warms considerably more slowly than the casing does. It takes a little while for the center of the cartridge to come to the same temperature as the chamber, and smaller cartridges warm faster.

If you'd like a shortcut to the data you're thinking of gathering, its in the article at http://www.shootingsoftware.com/tech.htm.

Blob1...

Nobody is disagreeing with what you say. But take it one step farther: When you first fire, the temperature of the chamber is determined by the ambient temperature. After you've shot several shots, and the firearm is hot, your fps will be higher still. The ambient temperature has not changed, yet you're getting more fps.

BTW, if you're that sniper, you have my respect, sir.