Velocity / Temp Formula

[tnichols]

Do you know of a formula/rule of thumb for velocity increase with temp increase? For example, my .223 was last chonoed at 12F. Today it's 47F here. I realize that re-chronoing is in order, but was wondering if you can use a formula/rule of thumb to compensate for the increase in fps to plug into Exbal for example. I was about 1.5 moa high today at 680 yards from my colder weather (temps in the 20's) data.

Load: 77gr. SMK over 24.6gr. of RE15 in prepped LC09 brass. Base to ogive legnth is 1.858. Fed GM205M primer.

24" barrel @12F = 2763 fps ES 22
Chrono: CED M2

 

[ChadTRG42]

Re: Velocity / Temp Formula

Rule of thumb is about .8 to 1 fps per degree. In a 308 with R15, the 1 fps holds pretty true.

 

[sermonator]

Re: Velocity / Temp Formula

Some powders are far more sensitive to temp than others. Moreover, many powerders don't seem to be affected as much in changes above 40-50, than they are as temperatures drop bellow that. I've done a little chronographing at temperatures around 30 and compared that to days in the 80-85 range. Most of the powders have been IMR varieties, except for RL19. My average difference was around 80 FPS for 55 degrees of change. This was also in cartridges of larger capacity that a .308, too. Just my experiences.

 

[tnichols]

Re: Velocity / Temp Formula

Thanks for the feedback. I've noticed here lately as we are starting to warm up that I'm running .5 to 1.0 moa high on the longer stuff (500+) and I was still using my data from when it was in the teens. Another thought is, with a gas gun, that second and subsequent rounds get loaded into a warm chamber and then sit while you are making corrections. Can't help but think that makes a difference too. If shooting a bolt gun, I usually leave the bolt back while I'm adjusting things, then chamber just prior to the next shot. Thanks again, I'm always open to suggestions. This is certainly a crazy game.

 

[Tripwire]

Re: Velocity / Temp Formula

Work up loads via OCW, and only worry about the outside extremes......

 

[tnichols]

Re: Velocity / Temp Formula

Don't mean to be ignorant, but what is OCW?

 

[varmint slayer]

Re: Velocity / Temp Formula

Optimal Charge Weight

 

[tnichols]

Re: Velocity / Temp Formula

Enlighten me on how this is different. I worked this load up in .2gr. increments until I achieved the fps I wanted, verified with ladder tests at 300+ yards and since then have focused on the shooting and not fussing over load development. I'm just trying to eliminate variables that lead to first shot misses. I appreciate your input.

 

[Clark]

Re: Velocity / Temp Formula

Kombayotch took an oven to the range to produce this data.

On an internet populated by posters who would not know a controlled test if it bit them, he is an oasis.

 

[wksinatl]

Re: Velocity / Temp Formula

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tnichols</div><div class="ubbcode-body">Enlighten me on how this is different. I worked this load up in .2gr. increments until I achieved the fps I wanted, verified with ladder tests at 300+ yards and since then have focused on the shooting and not fussing over load development. I'm just trying to eliminate variables that lead to first shot misses. I appreciate your input. </div></div>

Sounds to me like you are GTG.

Keith

 

[pdogdown]

Re: Velocity / Temp Formula

Might want to check Hodgdon's website and read about the extreme powders(i.e. Varget,H435,H4831SC, etc. and IMRs XBR 8208). I can't stand RL powders. They can shoot ridiculously good one day and 20 degrees and a little pressure difference and they get all over the place ala RL17 and RL15. It can be maddening. I don't have the time or patience to constantly "tune" loads for the conditions. I probably wouldn't excel at the BR game

Justin

 

[Tripwire]

Re: Velocity / Temp Formula

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: tnichols</div><div class="ubbcode-body">Enlighten me on how this is different. I worked this load up in .2gr. increments until I achieved the fps I wanted, verified with ladder tests at 300+ yards and since then have focused on the shooting and not fussing over load development. I'm just trying to eliminate variables that lead to first shot misses. I appreciate your input. </div></div>

Sounds like your main goal was achieving a certain velocity, but you neglected to determine if that velocity was in an accuracy node according to the principles of OCW. Meaning a range of velocity, and thus a range of pressure, that produces the same POI at any given range. Working within a range of pressures, with the middle of that range as the standard load, negates a lot of variable found in changes of ambient temperature.

 

[Fred_C_Dobbs]

Re: Velocity / Temp Formula

FWIW, Temperature Correction Chart Used by Les Bowman