IMR 3031 and Temperature

[Ryan500]

I was at the range today, about +10 degrees and my groups that were about 1.5" at 100 yards (-15 to -20 degrees) are now about 4 or 5". Do you guys change loads for different temps? I have been googleing a bunch and see that some do some don't. Some powders are temp sensitve, some are not.

My load if 42.8gr IMR 3031, 150 gr Hot-cor SP, for a 7.62x54r. OAL 2.900".

I have heard/read that IMR 3031 is good for temp changes so i'm wondering what the masses do? I will be playing with my loads some more regardless...

Thanks, Ryan.

 

[kombayotch]

It isn't bad for an older powder. Here is some data from a test I ran with it in 308 Win.:

Here is a Varget load for comparison:

The number in front of the 'x' is the slope in fps/F. So, IMR3031 changed 58.8 fps over 100F, while Varget only changed 14.9 fps over 100F.

If velocity changes, pressure changes and so does barrel time. So, yes, loads to get re-tuned over temperature if you aren't using a temperature stable powder.

 

[28 gauge]

Could be your barrel is warming up quicker on the hotter day causing the groups to move.Try waiting longer between shots.

 

[mihasic]

IMR 3031 is one of my favorite powders. Never saw any temperature problems with it. I shot it from -10 to +30 C. I use it for light bullets - just 110gr in 30-06. Normally under 1MOA 5-shot groups at all temperatures.

 

[thebigslide]

I have found that IMR powders are more sensitive to temperature change than their hodgdon equivilents.

Filler may help you out as I have found IMR 3031 likes to be compressed for consistency. I think it helps make for consistent flame propogation within the case.

Try reloader 7 and H322. Both of those powders have done me very well as far as consistency in various temperatures. The new Alliant Reloader 7 is much better than the old Herculese.

 

[kombayotch]

I haven't found any of the Alliant powders to be temperature stable. I would go with H322.

 

[28 gauge]

I have used IMR powders for years.Never had any problems of any kind with IMR powder.

 

[Winnipeg Toymaker]

I use IMR 3031 to reload for all my guns calibers (13) from .222 thru 45-70. I have never encountered a problem whether hunting at +30 or -30....the bullets fire first time...everytime. The colder weather may have a determining factor on your bolt and firing pin perhaps.

 

[Northman999]

I load my dad's 375 H&H with 3031, as he got a great deal on several pounds of it. We have not noticed any practical difference, whether he is hunting mooose at +15, or Bison at -15.

 

[Northman999]

It isn't bad for an older powder. Here is some data from a test I ran with it in 308 Win.:

Here is a Varget load for comparison:

The number in front of the 'x' is the slope in fps/F. So, IMR3031 changed 58.8 fps over 100F, while Varget only changed 14.9 fps over 100F.

If velocity changes, pressure changes and so does barrel time. So, yes, loads to get re-tuned over temperature if you aren't using a temperature stable powder.

This testing was theoretical and done on a computer, right?

I mean, you didn't actually find somewhere to fire these loads at +175 degress???

 

[kombayotch]

No, I did this on a rifle equipped with a Pressure Trace System in real life. The x-axis is the ammo temperature in degrees F, not C.

What people notice based on air temperature isn't the best indicator of temperature sensitivity. Rounds brought from home in a plastic case, transported in a heated/cooled vehicle will often experience very little change in temperature internally. Unless the rounds are soaked in an exposed manner, at temperature, for at least two hours, you don't really know the powder is at that temperate. The first time I ran this type of test, I put a thermocouple inside a loaded round (without primer) through the primer hole to see how long it took for the internal temperature to be the same as external.

The way the tests works is that you soak 3 batches of ammo at hot, cold and ambient temperature for at least two hours (5 rounds of each). I use a cooler with a heating pad for hot, the freezer in the clubhouse for cold (an icebox works too), and just sit the rounds out in the open for ambient. Temperature is measured with thermocouples and/or mercury thermometers.

You then fire them one round at a time in a round robin sequence (ambient, hot, cold, ambient, hot, cold...). You let the barrel cool in between each shot. When you fire each round, you just transfer that round from the hot/room/cold storage to the chamber and fire it through the chrono as quickly as possible. You don't want to give the rounds any time to change temperature internally due to contact with the chamber. I set the scope on the lowest magnification and just make sure I'm sending the round through the sweet spot of the chrono (no groups here). You basically want to the Jerry Miculek of loading a boltgun and firing it through the chrono. This is best done on a short pistol range, close to the berm.

If you're interested in the powder's temperature sensitivity, you don't soak the entire rifle, that isn't scientifically correct because you're mixing multiple effects. Thermal expansion/contraction of the barrel due to temperature is a different effect than powder temperature sensitivity. In this test, we are only interested in the powder's effect (primer too, as it cannot be isolated). By using the round robin sequence and letting the barrel cool in between shots, you are removing its temperature effect as a variable in the test. If you then wanted to take it to the level of testing it as a system, you would know how much the powder is contributing. Otherwise, you cannot isolate it from the barrel's effect. Then, you don't know how much each is contributing to the problem.

It isn't a difficult test to run. However, if you don't have the ability to measure pressure, I would advise putting the heating pad on a lower setting.