Temperature vs. FPS?

[nhglyn]

A basic question........

If I load my 38 Super here in cold New Hampshire for 1350 fps (and about 170 pf) when chronoed and then go to Florida for a match, what is the resulting change in fps and pf if there is one at all? Does fps go up in hot weather? Down? Why? I would think that a specific powder charge made for 1350 fps would chrono the same anywhere......but what do i know?????

[Flexmoney]

Many factors. Biggest being the powder you use.

I think the VV manual (not the phamplet) has data that helps for their powder?

[dillon]

It depends purely on the powder you are using. Some powders, such as W-231, the pressure and velocity go down as temperature decreases (ask anyone that shot the USPSA Limited Nationals in Reno, NV a few years ago!) Other powders, such as Blue Dot, go up in pressure and velocity as temperature decreases. Best bet is to chill some ammo, chrono it, and warm some ammo and chrono it.

[schmitz]

In their Reloading Manual VV states that: and I quote: "The influence of powder temperature on the burning rate and thereby on the chamber pressure and muzzle velocity is remarkable. VV powders have an experimentally temperature gradient of appprox. 3% change in the chamber pressure and 1 % change in the muzzle velocity per 10 degrees C change in powder temperature..."

VV Reloading Manual, 1995, ISBN 951-96950-1-X

[SRT Driver]

Case in point, Open pistol VV N-350:

10 Degrees F = 1325 FPS

75 Degrees F = 1390 FPS

[nhglyn]

Thanks everyone, that's the sort of information I was looking for. You've been great.

[mcoliver]

I'm almost afraid to task this for fear of ridicule but I find comfort in my flame retardant suit:

How do you take the temperature and from which location (chrono, gun, ammo box, ...somewhere)?

[SRT Driver]

My temperature readings were taken with a thermometer I thru in my bag. Not scientific, but neither am I. A loading manual said the "lower pressures and velocity result from lower temperatures." I wanted to know if my loads would make PF in summer and winter.

I wanted to duplicate match conditions. The ammo went from the trunk into the clubhouse and out to the range while I set up the chrono etc. The ammo was not 10 degrees but niether was the pistol after a few shots. Same in June at Area 5.

The point was to see how much the air temp and density changes the velocity. And this was over a several month period. (Jan thru June)

It was strange that 1 degree = 1 fps. Another finding, 231 is very temp sensitive, VV was not (N320 minor, N350 Open Major)

[mcoliver]

Oh I see. Thanks.

[sargenv]

Other powders, such as Blue Dot, go up in pressure and velocity as temperature decreases.

I'm kind of surprised about this. Maybe it has to do with the fact that we are dealing with pistol pressures here. In shotshell reloading, Blue Dot is efficient

in temps above 50 degrees, but becomes sluggish at lower temps (<40 degrees)and doesn't generally get as clean a burn. I'm not going to make direct quotes, but Ballistic Products provide loading data that suggests that slower burning powders generally become less efficient as temps drop and therefore pressure and velocity are affected. Generally we would be talking about Blue Dot, SR 4756, and 571/HS7. This doesn't apply to some more modern propellants such as Titegroup, and any of the powders designed not to be affected by temperature variations.

They actually have specific shotshell loads utilizing generally mid to faster burning powders that at normal temperatures would be over pressure (over 50 degrees F), but at the temps they advise, you will get a better burn and good velocity. The powders that come to mind are specifically Green Dot and PB. These are for temps less than 40 degrees F.

Anyway, I thought I'd chime in with that little bit of curiousity.

Vince

[kevin c]

Remember that air temp is just where your ammo starts out. A box of ammo left out in direct sun will be considerably hotter, both actual ammo temp and chrono result wise, than a box kept inside your range bag.

I've also found that a round cooking inside the chamber of my gun just after a rapid fire string is hotter over the chrono than the first round out of the same gun shot cold (by way of example, the first round in any string fired with my VV320 match loads is nearly always the lowest velocity of the string by a considerable amount).

[shred]

7625 is definately reverse-sensitive. A 40'F temperature drop adds 30+ fps to some of my Super loads.

The canonical way to test the cold-weather performance of ammo was posted many years ago to the IPSC e-mail list. I haven't got a copy, but it goes something like this:

Get an Eskie (styrofoam cooler), fill it with ice. Put in a beer or two to keep the ice from rattling around. Put some of your ammo in a baggie and put it in the cooler. At the range, pull the ammo out and chrono it, compared to normal-temperature ammo. Put the guns up and drink the beer