VV320 and Temperature

[Mistral404]

I thought I would share some observations.

I have been reloading my .40 using VV320.

Fairly new Nowlin barrel in an SV extended frame.

Dillon 1050, every third round was weighed.

The loads:

a: 5.0 gr. VV320, 1.19 aol, 180 MG JHP

b: 4.8 gr. VV320 1.19 aol, 180 MG JHP

Yesterday it was very cold, around 35 degrees at 9 am. (ammo left in garage-very cold)

a1: 836 fps 150 PF

a2: 839 fps 151 PF

a3: 841 fps 151 PF

a4: 840 fps 151 PF

a 5: did not record the fps 152 PF

b1. 890 fps 160 PF

b2: 878 fps 158 PF

b3: 858 fps 153 PF

Today, temperature around 53 degrees at 1pm ( ammo warmed by sun)

a1: 919 fps 165 PF

a2: 910 fps 164 PF

a3: 930 fps 167 PF

b1: 953 fps 171 PF

b2: 947 fps 170 PF

There were anywhere from 5 to 10 shots per grouping.

There was significant increase in velocity when the weather got warmer.

Both days used ammo that was reloaded at the same time.

[kevin c]

I have found that the first round of my N320 match loads, shot out of a cold barrel, is almost inevitably the slowest in the string. As subsequent rounds in the string warm up in the chamber, they shoot faster. This applies all year around here in California (range of 45 to 90 degrees Farenheit).

Another reason to run a little hot for major matches, since that first round is, for me, almost always the throwaway round on velocity.

[D.Hayden]

The loads:

a: 5.0 gr. VV320, 1.19 aol, 180 MG JHP

b: 4.8 gr. VV320 1.19 aol, 180 MG JHP

Are these reversed?

[Mistral404]

GOOD CATCH!

Yeppers they are reversed.

"a" should be the 4.8 gr

"b" should be the 5.0 gr

btw: frequently the first round of any group was the slowest.

[.40AET]

Dissapointing news at best.

[ChuckS]

God, I love this kind of crap! I am an engineer by trade and typically overcomplicate anything thing I touch. This subject may be the newest. First of all, I listed all the stuff that could be affected by temp.

1) Powder performance vs temp

2) Primer Performance vs temp

3) Crimp (bullet and case temperature coefficient (TC))

4) Chamber fit (case TC)

5) Bullet to barrel friction (bullet and barrel TC)

6) Air density

7) Accuracy of the chrono vs. temp

There are probably more but this is my starting list.

Looking back at the list, I am not sure about 1 since the Vit (free) manual does not discuss temp effects. Not sure about 2 either. 3-5 are probably pretty small but those may be the answer to the cold first shot thing.

I found some interesting stuff at http://www.varminthunter.org/ballistics.shtml

First it shows:"A major powder manufacturer, DuPont, says that with smokeless powder, a velocity drop will occur below 70 degrees Fahrenheit of 1½ fps per degree."

You had an 18 deg. difference so that give a 27 fps drop. That could account for 36% of A and 28% of B.

There is still a lot to explain. The colder air is more dense. In fact, referencing another part of that web page, it looks like you have the equivalent of ~1000 feet altitude from 35 to 53. There is also humidity that comes into play. See if you can find a bench rest shooter who can give you some idea on the magnitude of the effects that density altitude have on bullets.

For 7, I looked at a couple of chrono web sites. No real accuracy information on most of them except for Oehler. The accuracy errors look pretty small. None of them mentioned accuracy vs. temp.

I am not really sure what all this means except that things will slow down as the temp decreases. Like I said, talk to a precision rifle shooter about density altitude effects on velocity. One thing for sure, only shoot major matches during good weather!

[shred]

I know we've gone over this somewhere else, but Chuck has a good list of possible error sources.

First it shows:"A major powder manufacturer, DuPont, says that with smokeless powder, a velocity drop will occur below 70 degrees Fahrenheit of 1½ fps per degree."

Some powders do, some powders don't. IMR 7625, for example produces more velocity when the temperature drops (these are the 'reverse temperature sensitive" powders).

There is still a lot to explain. The colder air is more dense. In fact, referencing another part of that web page, it looks like you have the equivalent of ~1000 feet altitude from 35 to 53. There is also humidity that comes into play. See if you can find a bench rest shooter who can give you some idea on the magnitude of the effects that density altitude have on bullets.

For pistol bullets and a chrono 10' away? The bullet loses between 1 & 1/3 fps per foot plowing through a standard atmosphere, but atmospheric differences will be negligable over that distance.

I am not really sure what all this means except that things will slow down as the temp decreases.

I check match ammo by sticking some in the range freezer and refrigerator before chronoing if it's a warm day, which eliminates #6 & 7, but I'd bet in this case most of the variance is due to the powder characteristics alone.

[ChuckS]

I check match ammo by sticking some in the range freezer and refrigerator before chronoing if it's a warm day, which eliminates #6 & 7, but I'd bet in this case most of the variance is due to the powder characteristics alone.

Shred,

You have a range fridge???? At Norco, all we have is dirt. Very dusty dirt in fact

What sort of variation do you see cold to hot? I am still thinking there is something else going on here. As you know, quite a few people used Vit and many of them tend to ride the 165 line fairly close. Having been burnt with someone else's ammo at my first sectional, I tend to target 170 (using N320 with a 180 gr plated bullet). The slowest I have ever chronoed was around 167. (Area 2, Mesa Az, Morning, November. Don't know the temp but many people were asking how I could be wearing shorts!)

Does anybody out there have the Vit reloading manual that you have to pay for? That book may have some info on them effects.

Happy New Year to All

Chuck.

(2005?? Damn, it seems like only yesterday we were hoarding .223 for the upcoming apocalypse...)

[shred]

I check match ammo by sticking some in the range freezer and refrigerator before chronoing if it's a warm day, which eliminates #6 & 7, but I'd bet in this case most of the variance is due to the powder characteristics alone.

Shred,

You have a range fridge???? At Norco, all we have is dirt. Very dusty dirt in fact

Well, then you need the classic Australian way of temperature-testing ammo..

"Get a small ice-chest and ice. Add bag of ammo. Throw in a beer or two to keep the ice from rattling around. Go to the range, chrono ammo, then drink the beer."

What sort of variation do you see cold to hot? I am still thinking there is something else going on here.

The same load of 7625 will go from 175 PF fresh from the freezer down to 165 PF at 90'F. I haven't got good data on N320, but what I do have shows not a lot of variation between 55'F and 90'F for my steel loads, maybe 1 or 2 PF, so you may be right.

[Detlef]

shred caught it, air density changes due to temperature variations have a (statistically) nonmeasurable effect on close-to-muzzle velocity (the kind we are concerned with). But long-range rifle shooters know the effect and applying POI corrections is essential to them.

--D.

[spook]

Cool thread, and thanks for posting your results Mistral. I started a thread not so long ago about the huge strange drop in velocity I had when changing my OAL just a tad with my .45 load and N320. The thing I realised when I read your post is that the last time I chronoed was in -5 C and the time before that in +25 C. Hehe, you could be on to something.