Hot temps and load reduction

My 6.5 load was pushing 3,000 fps when it should have been closer to 2,700. I've dropped 2.5 grains (just over 5%) and will see where it takes me this week.

 

I would go back to H4350.

I also shoot 6.5 Creedmoor. I just got back from the range where it was 90+ and full sunshine. My velocity was up less than 10 fps over 70 degree average. Worst round was 16 fps over according to my Labradar. I am shooting H4350 and 140gr Berger hybrids.

 

tc2017
How are the grouping at what range , I'm shooting a 308 the 6.5 CM is a flat shooting round pretty far out there .

 

my groups at 200 yards are fine. The problem was tough extraction and primer cratering, though bolt lift was normal. I haven't had an issue with my .308, but then I'm mostly loading Varget, H4895, or RL15.

 

I work up until I just begin to see the first signs of pressure and back off 2% from there. It keeps me out of trouble when the weather heats up, and is generally in the immediate vicinity of the middle node.

 

I saw a chart somewhere that identified the velocity increase per grain of powder of various different powders. I may be off the mark, but I would assume the pressure increase would follow the velocity somewhat linearly. Finding that chart and choosing your powder may be a good place to start.

 

Pressure and velocity are often only perfectly linear within a relatively narrow range of charge weights. This is especially true at (or near) MAX pressure, where things can skyrocket very quickly with only a tiny increase.

Although it's possible to make "guesstimates" from publicly available temperature-dependent burn rate data for various powders, or to use a Program like QuickLoad, they may not be all that good for a variety of reasons. Your best bet is simply to determine velocity with a known charge weight at different temperatures through a shooting season. Doing this will give you the data you need to appropriately adjust cool weather loads (if necessary) for hotter temperatures or vice versa, in your specific setup.

The simplest approach is to adjust charge weight to maintain the velocity of a load that is known to be working well within a given temperature range as ambient temperature fluctuates. For example, you work up a good load that chronos at ~2850 fps when the ambient temperature in the late winter/early spring is hovering around 40 degrees. Later in the summer when the temps are in the low to mid 90s, you find the load chronos at at 2890-2900 fps and doesn't have the same accuracy/precision it originally did. Decreasing the charge weight until the load is back to the original ~2850 fps at the higher temperatures will usually get it back very close to where it was originally.

 

Changing powder load can of course restore MV, while also affecting load density. But remember that there is still barrel temp differences to deal with. That is, for hunting (not BR).
The folks behind Pressure Trace compared the 4350 powders, and with it there was a bit of discovery about barrel temperature influence: https://www.shootingsoftware.com/ftp/Pressure Factors.pdf

Their conclusion:
- Both barrel temperature and powder temperature are important variables, and they are not the same variable. If you fail to take barrel temperature into account while doing pressure testing, your test results will be very significantly affected.
- If you’re not controlling barrel temperature, you about as well might not bother controlling powder temperature, either. In the cases investigated, barrel temperature is a much stronger variable than powder temperature.

 

Yep, just lower the charge to match your winter fps.

You can keep the ammo in a cooler and out of the sun.

Ha, be a fair weather shooter like me, no brain cooking or ass freezing going on here! I live 2 hours from Phoenix AZ, no friggen way I'm going down there in the 3 hottest months and no way I'm going shooting when it's less than 20 up here where I live at 7000ft.

I use temp insensitive powders and only adjust .1-.2 tenths from summer to winter depending...

 

It’s so damn easy, why people make things difficult, leave ammo in cooler until you need it. Gee whiz!

 

I have a temp strip on all my barrels, and stop shooting when it hits 120*. The issue I had with RL17 and my 6.5 was from the git-go, cold barrel. I'm going to not use that powder during the summer and stick to H4350 where I have not had issues.

 

I live in the desert and have really struggled with hard bolt lift. I have finally learned to go much lower. Chasing the top velocity node no more! I'm much lower now. 28.2 of Varget, 107 smk in my 6br. Shooting a 600 yard f- class match the wind may push me around a bit more, 2". But misjudge the wind by 1 mph and the 2" is gone anyway. I can't read a 1 mph wind change but I will continue to practice alot without worrying about pressure. I've drank the top velocity coolaid enough. Mike.

 

You may laugh

You may laugh at me but I keep my loader ammo in the freezer .For summer shooting. I live in San Antonio. and it is hot in summer.I take it to the range in a small lunch box on ice.. Just my two cents.. Tommy Mv

 

Big Mc: I'm not that far north of you - near Killeen in Bell County. No laughs...you aren't the only one who has suggested keeping ammo cooled. I've thought about taking my ammo to the range in some sort of cooler on top of blue ice. I'm sure I'll get some funny looks, but what the heck if it works! May try it this week.

 

Keeping ammo chilled may help a little bit. However, a significant amount of the powder burn occurs in the first few inches of barrel. Once the barrel heats up, simply keeping the ammo itself cooled isn't going to completely prevent temperature-dependent velocity increase. Adjusting the charge weight by up to a few tenths in either direction to keep the velocity constant across the temperature range you're likely to encounter during the course of a season really isn't a big deal.

 

Thanks, Ned. Actually my thought was a combo of keeping my ammo cooler plus lowering my charge weight. I haul enough stuff to the range without adding anything too large. I was thinking an MTM box with blue ice in the bottom...

Its an interesting discussion as it is almost the opposite problem for rimfire in the winter. There, cold ammo (shooting in conditions under about 35 degrees) will see a POI drop of about 4" at 100 meters (shooting silhouette...) vs a summer POI. I used disposable handwarmers in my shooting bag to keep my ammo warm, and when I had loaded magazines I kept them in a pocket with a handwarmer and never had a problem. I tried biathlon ammo (slightly hotter powder) which helped, but the ammo was pricey. as my hands got cold too, I killed two birds with one stone.

 

If you lower your charge and work up in the temps youll be shooting in theres no need for ammo chilling. Thats a dangerous band-aid

 

Dusty: is your reference "That's a dangerous band-aid" to chiiling ammo?

 

If you have to chill it to keep it from being over pressure in the actual environment youre shooting in then yes its a band-aid and its a dangerous situation

 

thank you Dusty!