Single digit SD’s

[BallisticGuy]

I’m finally there with my hot weather load for my 6.5cm. I had to back down .5 grains of H4350 on my go to 600yd load to get me back into the node when air temps are above 95 degrees. I did my load development when it was 68 degrees, and my groups open up and gained roughly 60fps with this load when it’s over 100 degrees outside.

I finally have two solid loads, needless to say I’m learning!

-41.2gr of H4350
-140gr ELD-M
-2.210 CBTO
-24in barrel

 

[swampratt]

I found letting one bake in the chamber on a hot day really adds to some opened groups.
It is amazing how hot one will be if you shot a few then chambered one and wait a minute and pull it out of the chamber.
Pretty toasty.

 

[kwaynem]

I get single SD’s with 41..0 grain of H4350 too hot for me to try in hot weather I weld in it all week I’m not getting in it on the weekends lol

 

[diggler1833]

This is pretty interesting. Most people swear by the temperature insensitivity of H4350, yet you are having to back off a half grain, so probably we're experiencing a 2 FPS / degrre swing.

In most of the tests I've seen conducted, H4350 is down around the .25 FPS / degree range...so theoretically whopping 25 FPS difference on a 20 degree vs 120 degree day.

My H4350 experience is pretty limited, but I did develop a load for my first 6.5x47L last month in 95+ degree weather too. I was using H4350 for it (see below).

I went with 39.4 and cut the difference between the targets on the left and center, and then shot a confirmation group at 385 yards (farthest I can shoot in a safe direction from my porch).

It'll be interesting to shoot this load in the winter and see how much velocity I lose. I'm pretty curious now that I read about you having to drop a half grain. It is something I expect with a powder like Re-15, CFE-223, TAC etc... but not a Hodgdon extreme. I have quite a bit of experience with Varget, and taking my M24R for example (7.62x51), my charge weight needs no change between 40 and 80 degrees...and unfortunately that barrel is pretty finicky about velocity.

Did you change powder lots by chance? I do know that at one point H4350 had a pretty significant swing in velocity, but that was years ago.

How is your cleaning regimen? Sometimes excessive copper, or even the dreaded donut will spike pressures and occasionally velocity.

Sorry for the question volley...something is just not adding up in my head how you can get a 60 FPS swing over a 32 degree change with one of the most (if not the most) temperature stable powders on the market.

Strange things happen though.

 

[swampratt]

I think some of the velocity changes we experience can be chalked up to how much air space is left in the case.

 

[magna19]

I think the powder manufactures use a formula for temp swings in powders and if under the number the powder is considered not temp sensitive but as most know when a round goes off temperature affects velocity to a degree regardless of powder.

 

[diggler1833]

I think some of the velocity changes we experience can be chalked up to how much air space is left in the case.

I would absolutely agree under other circumstances...but in this instance, his case fill should be above 90%, and his ES and SD are pretty good, so that would just about naturally rule that out.

Traditionally though, yeah...an 80% case fill will usually result in crappy ES and SD unless one lands on a miracle node.

 

[diggler1833]

I think the powder manufactures use a formula for temp swings in powders and if under the number the powder is considered not temp sensitive but as most know when a round goes off temperature affects velocity to a degree regardless of powder.

I bet you can be right on that. Typically though, single-based powders are going to be more temperature stable, usually the cost of reduced velocity.

My point earlier was based off of relatively controlled, third-party testing like in the link below:

Powder Temp Stability: Hodgdon Extreme vs. IMR Enduron

The goal of this field test was to quantify the temperature stability of the popular Hodgdon H4350 a...

precisionrifleblog.com

 

[magna19]

I would absolutely agree under other circumstances...but in this instance, his case fill should be above 90%, and his ES and SD are pretty good, so that would just about naturally rule that out.

Traditionally though, yeah...an 80% case fill will usually result in crappy ES and SD unless one lands on a miracle node.

I agree. Low SD is low SD. I think he may have meant to say air space position in a cartridge creating different velocity. Who knows lol

 

[diggler1833]

I agree. Low SD is low SD. I think he may have meant to say air space position in a cartridge creating different velocity. Who knows lol

I remember reading an article about crazy velocity swings in the .45 Colt, which apparently was completely dependent on how the revolver was handled (with the cartridge in it) before it was fired. You'll have to forgive my fading memory, but I think there was a significant velocity loss when the revolver was kept level...and the powder was presumed to be laying much more loosely in the bottom of the case. That was vs. the revolver being tilted upward to settle the powder closer to the primer before it was fired.

I'm going to see if I can find that article online. It would definitely support his post...but rifle cartridges usually have a lot more case volume used up with a powder charge.