Temperature efftect on powder MV and where to correct?

[EXPRESS]

After testing my latest load I have found I get about 2.6fps variation in MV per one degree celcius of temperature drop.

The accuracy load I have at the moment shoots very well at 2485fps, but what I want to find out is; is it better to adjust your trajectory to the temps, or adjust your load to the temps?

In this case I developed the load in warm temps, 27C, but my hunting will probably be in temps from 0 - 15C. That means a drop in MV of around 50fps. If I add 1.5grains of powder I know I will be back around 2485fps. But in todays temps, that puts me out of the accuracy node.

The ideal would be to do your load development and zeroing in the same confitions to your hunting, but when that isn't possible, I wonder which option is going to maintain the accuracy?

Surely it's the powder (MV) and not the correction?

 

[Mikecr]

It's both.
Correct both MV and air density factors.

 

[EXPRESS]

I mean; add or detract powder and make no mv correction, or make mv correction maintaining the same load.

My test today with three rounds warmed up to 37C (99F) showed the accuracy was lost. MV increased by 3fps per 1C.
This indicates that it is best, if you know how and have the chance, to adjust your load to the temps.
Next trial I will add 1.6 grains of powder, then cool the cartridges to 5C, which should give me 2485fps and .5MOA.

 

[FAIRCHASE]

Reminds me of a dog chasing his tail.

 

[Ridgerunner665]

Reminds me of a dog chasing his tail.

Good way to put it...


I find one load that is accurate enough (no more than .5" to .75") across a velocity span of 100 fps in either direction...I haven't had much trouble finding such loads...even with Norma MRP (Reloder 22).

Then I input the fps/degree deviation into my ballistic app (AB in my case)...

A load that has a very narrow accuracy node is of little use to a hunter in my opinion...conditions change too fast, and hunters don't generally have the stuff with them to load more ammo on the spot.

Don't push the limits on pressure...my one exception to that rule is Reloder 26, good stuff at 58-60k psi...and its even better at 63-65k psi...deviates less than 1 fps per degree in the guns I've used it in (270 Win and 280 Ackley)

 

[Mikecr]

I mean; add or detract powder and make no mv correction, or make mv correction maintaining the same load.

My test today with three rounds warmed up to 37C (99F) showed the accuracy was lost. MV increased by 3fps per 1C.
This indicates that it is best, if you know how and have the chance, to adjust your load to the temps.
Next trial I will add 1.6 grains of powder, then cool the cartridges to 5C, which should give me 2485fps and .5MOA.

Well, you need your load to be in tune, so you will need load development for conditions. Whatever you end up with for MV and actual temperature, these both need to be accounted for in ballistics.
I don't assume your best load will hold over a big range in temps(mine do not). One thing that helps me is to actually heat ammo. If I could, I would keep it at ~110deg in the field, but I can't right now. What I do instead is store ammo I'm about to use, and during load development, in my front pants pockets. That's right, next to my balls, which are stable in temps all year round. A gun can change with temps as well, but at least my ammo doesn't. The reason I choose heat over cool(like using a cooler) is because powder varies less at higher temps than cooler temps. From 100 to 120, most powder barely varies in burn rate. But go from 60 to 80 and look out.

 

[EXPRESS]

I'm not suggesting changing loads on the spot, it's that my testing, which while scientific in approach is only a single case, so statistically insignificant, I have seen that the speed at which the bullet travels down the barrel is what determines the "accuracy node" and not the quantity of powder burned.
A load which shoots accurately 100fps either side would be truly awesome! But if you consider I found 112fps difference from 37C (99F) to -20C (-4F) it's unlikely that you will ever encounter those two extremes unexpectedly.

I have always found my most accurate load, then adjusted for the temperature wherever I went hunting or shooting rocks. However this experiment has opened my eyes to something new, that those painstakingly acquired accurate rounds might just go to hell when you take them to the mountains or desert for your hunt, because every 13 deg C increase in temp has the same effect as adding/subtracting a grain of powder. And this is a "temperature stable" propellant!

The testing method I used was to put my rifle and ammo into a chiller at 2C, fire one shot over the Magnetospeed, then put it all back to return to temp and fire another shot, with three cartridges for each temp range. I used a freezer for -20C, which held my rifle too, and put the rifle in the sun measureing is temperature with an infrared thermometer, while the ammo I heated by putting the rounds into individually sealed zip loc bags into a bath of water with a sous vide cooker! Those things hold temp to a tenth of a degree.

It seems like this experiment gives us a reason to orefer the OCW method for finding our hunting loads because then we choose the most consistent bullet placement over pure group size, which is what we need.

If I get the chance today I will setup the final trial, to see if I can load to the requires speed at a determined temperature. In this case it should be by adding 1.6 grains and cooling the ammo and rifle to 5 deg C.

Once I've finished I'll write the and describe the whole thing in detail.

 

[Mikecr]

I have seen that the speed at which the bullet travels down the barrel is what determines the "accuracy node" and not the quantity of powder burned.

There are 3 broad nodes worked to our benefit, affecting each other, but independent still:
Powder (OCW)
Barrel (ladder)
Pressure (for competitive underbores)

The then are adjustment nodes:
Seating depth
Primer striking

Changing powder amount also affects load density, which directly affects burn rate. So these changes might not go as calculated. Well, this is why we test & validate.

 

[Wachsmann]

Don't push the limits on pressure...my one exception to that rule is Reloder 26, good stuff at 58-60k psi...and its even better at 63-65k psi...deviates less than 1 fps per degree in the guns I've used it in (270 Win and 280 Ackley)


Ridgerunner.....Interested to here more about RL26 and what large calibers you been using. Reasoning being yesterday I was out testing some loads. I've been trying to get RL33 at mag length to shoot on my 300RUM. Just not happing....I went with retumbo and it was better....I also had some RL26 and man it shot very well and put up some great number on the chrono as well. Also single digit SD number also. I was so impressed the last bullet I save out of the group to take home to make sure I try and duplicate the seating depth again. If this load hold true and not just a fluke then it will be my go to load for my RUM.