Load Development in High Ambient Temperatures

[daved]

I tried to do an OCW (H4350/6.5CM) yesterday when the ambient temp here in the NV desert was mid-high 90's. I've done OCWs in the past when temps where more "normal" (60-80) and not encountered any problems related to environmental conditions. I was blown away yesterday by the inconsistent POI and MV (LabRadar) of the test groups - the POI of some shifted anywhere from 2-4 clicks left and only one of the groups was near the size (< 0.5 moa) I normally shoot with this zeroed rifle in 75 degree temps. The MV of the load I normally shoot (41.7) was 30fps faster though the SDs of all the charges ranged from 2-12, eight single digit averaging 5. The only explanation I can come up with is mirage and probable increasing temp of the ammo as it progressed from room temp (75) to ambient (95). So the question is: how do you deal with high temps and the resulting mirage and ammo temp changes to get an accurate OCW.

Thanks for any input from desert dwellers and others---

Dave

 

[CharlieNC]

Dave the mirage at my range is usually bad too. I do load development at 200 yards because the POI movement is much more pronounced than at 100 yards. But the ability to clearly see and hold on target is not significantly worse at 200 vs 100, whereas stepping out to 300 gives a major deterioration in sighting. As a result the increased movement in POI vs no loss in sighting has always made node identification very clear at 200. For the sake of timing I've done this at 100yd and to me the outcome is just not as magnified.

 

[Fursniper]

I'm in the Arizona desert. I do load development early when it's still cool and use Hodgdon extreme powders whenever possible. If I'm going to be out when it's REALLY hot, I keep my ammo inside an igloo cooler to avoid excessive exposure to sun & heat. It all adds up and makes a difference....

 

[lash]

I'm in the Arizona desert. I do load development early when it's still cool and use Hodgdon extreme powders whenever possible. If I'm going to be out when it's REALLY hot, I keep my ammo inside an igloo cooler to avoid excessive exposure to sun & heat. It all adds up and makes a difference....

I've done the same thing here in Florida. I don't cool the ammo, I just try to keep it from getting overheated. Especially during load development and for ELR shooting.

 

[spife7980]

Go in the early morning. Keep it shaded.

I find I get sweaty and pissed off long before my ammo heats up making any results less than reliable lol

I thought h4350 was supposed to be the best for temp swings though.

 

[daved]

Well, I will definitely be doing load testing at a cooler time of the day in the future. It may be a dry heat, but it still takes a toll after a couple hours!

The idea of a non-cooled cooler also sounds good - just keeping the ammo in it until ready to shoot should keep it more uniform.

Thanks for the input--

 

[daved]

Go in the early morning. Keep it shaded.

I find I get sweaty and pissed off long before my ammo heats up making any results less than reliable lol

I thought h4350 was supposed to be the best for temp swings though.

I've wondered about temp sensitivity. It doesn't seem reasonable to expect the laws of physics to change - gas contained in the same volume (Chamber) will a have higher pressure at higher temp.

 

[Milo 2.5]

Aren't you really discussing 2 different things here? If your load is safe and productive in higher temps, now you need to adjust for the swings, apps help here, but a Kestrel 5700 combined with features in Kestrel Link Ballistics should negate your issues.
Trying to group past 300 yards in heavy mirage is futile if you're not used to it, even then can be iffy>not claiming I can, lol

 

[Deadshot2]

Temperature "insensitivity" doesn't mean "no change" merely less change. I know that Hodgdon touted H-4350 as only having single digit speed variations over a 0-100+ change in temp but I read all the time that people are seeing 25 or more fps change with their loads over temp variations similar to what you stated.

It would only stand to reason that when the powder is hotter any reaction will become faster. I don't have the 95 degree temps that AZ has to deal with but when it's 80 degrees here I don't close the bolt and chamber a round until I'm almost ready to pull the trigger. No waiting for wind or mirage with a chambered round.

I'm also with those that say "shoot early". I hate the freaking heat so by noon I'm either taking a nice A/C'd drive or back at the house enjoying some cool air that is created by a machine that is increasing my carbon footprint. Only need to do that for about 30 days per year, the rest of the time it's all about staying dry.

 

[Bbear2]

My understanding is that temp change varies according to the case size. Different sized cases will show different amounts of change per degree. Correct me if I'm wrong. In my part of Texas, one of the local ranges doesn't even open until 1 PM so, shooting when it's 'cooler' means 96 degrees vs 106. On those days I will put the rounds I'm testing in a little cooler with some ice blocks or similar to keep them cooler. The problem comes in trying to cool the barrel down below ambient temp. I've used the spray computer cleaners before and run a dry patch through after spraying.