Range Report Figuring FPS change as a function of temp

[tuckner]

What is the best way to figure out what your particular load's velocity change as a function of temperature. The end data would tell me 1.4 FPS per degree (F) for example.

Would I just chrono at a high temp, and then a low temp and divide the FPS change with the temp change and would that be an accurate enough calculation? If so, what would be the best baseline temp difference, 50 degrees or more, or is something like 20 degrees adequate?

Probably a simple question, but I just want to be sure.

I am trying to gauge this for 175SMK and 168SMK for use with the FDAC to ensure I have the right card installed, and then to put a printed label on each card that would say 175SMK 75 to 100 (F) and on the same card 168SMK with it's temp range for the velocity to be close to the charts.

Thanks for any useful response in advance.

 

[Lindy]

Re: Figuring FPS change as a function of temp

20 degrees is not sufficient. I try to get a temperature close to freezing, which I can only do down here in Texas in the dead of winter, and 90 degrees, which I can do much of the year.

And, yes, just take the difference in velocities and divide by the difference in temperature.

However, absent the ability to do that, if you use 1 fps per degree F. with most modern powders, you won't be wrong by very much. (That does <span style="font-weight: bold">NOT</span> apply to the old military A191 .300 Win Mag load.)

If you look at this page, you will note that one rule of thumb is to change the DA by 500 feet for every 15 fps change in muzzle velocity.

Making Ballistic Cards Using Density Altitude

If you really know what the change in velocity is, though, you can use JBM Ballistics DA calculator to get a closer figure.

 

[tuckner]

Re: Figuring FPS change as a function of temp

Okay, I will just go by the rule of thumb of 1 fps per degree F until I can get something below freezing this winter to confirm. I am using FGMM 168's and Southwest Ammunition M118LR, which I suspect holds true close enough to the rule of thumb, but just want to be sure.

Thanks for the reply.

 

[CoryT]

Re: Figuring FPS change as a function of temp

You can leave a baggie with test rounds in the freezer overnight, then transport in some salted ice water in a cooler. Leave a thermometer in the water to confirm the ammo temp, load and shoot within 1 min and you'll be good to go. To get hotter than ambient, wrap the rounds in some chemical handwarmers, again with a thermometer to get the test temp. They need to be in the wrap for at least 30-40 min for the core temp to settle.

The 1 to 1 is pretty darn close for most current commercial powders excepting the Extreme line from Hodgdon, certainly well within the SD of the load in any event. Both Varget and Retumbo work out to 1fps for 11 F for my loads.

 

[Lindy]

Re: Figuring FPS change as a function of temp

Having the barrel/chamber and the ammo at different temperatures are likely to lead to significant errors. I don't regard that kind of testing as worthwhile. For why, see:

http://www.shootingsoftware.com/ftp/Pressure%20Factors.pdf

Denton wrote another article as a sequel to that one, which showed data which indicated the barrel temperature was even more important, but I cannot find it online any more. At one time, it was posted on this site.

 

[tuckner]

Re: Figuring FPS change as a function of temp

Although interesting idea to freeze and heat the rounds, I would rather they all be ambient temp with the entire system as a whole for redundancy.

 

[MontanaMarine]

Re: Figuring FPS change as a function of temp

FWIW, My go-to 30-06 load with RL22 powder chrono's 2720 fps at around 75F.

I let the rifle and ammo set outside one winter night when the temp went well below zero. Next morning at -21F I chrono'd it at 2620 fps.

Lost 100 fps in a temp swing of 95 degrees F. Couldn't say if the drop is linear with temp, or on a curve. I should check it at about 25F, and see what I get.

 

[tuckner]

Re: Figuring FPS change as a function of temp

I would figure it is linear, would be interesting to know for sure. That would be a pretty consistent drop for such a swing, just about that 1 fps per 1 degree as stated above (with modern ammo).

 

[Sniper1*]

Re: Figuring FPS change as a function of temp

FWIW, Here are the numbers for my Hornady 110gr TAP Factory ammo.
I sent a box to the FBI Ballistics Research Lab and they found about the same results under extremely controlled conditions.

11*- 3129 fps
26 - 3158
74 - 3299
90 - 3318

I have yet to get a good temp range for my 168 TAP Amax, so I will be doing that soon with the seasonal change coming!

 

[tuckner]

Re: Figuring FPS change as a function of temp

2.39 fps per degree on that load seems a lot. Well, I'll just have to use the rule of thumb of 1 fps per degree but definitely get out and confirm both loads.

1.93 fps per degree from 26 to 11 F
2.93 fps per degree from 74 to 26 F
1.18 fps per degree from 90 to 74 F

If my impromptu math was correct seems that between 74 to 26 degrees there was a bigger change in the stability of the powder with regards to temp. Pretty interesting.

 

[Sniper1*]

Re: Figuring FPS change as a function of temp

Yep! I even forwarded the info to Hornady, who still says their powder is not temp sensitive!

For Me, Temp is a much bigger factor in long range accuracy than anything else. I have to figure out the MV @ temp then adjust dope accordingly.

My Horus Vision programs did that by interpolating the MVs by Temp but it has kind of become obsolete compared to the Iphone apps and such.

I was keeping in close contact with Horus but that communication line dropped of the face of the earth when the Military contracts started to heat up.

Maybe the Kestrel/Horus unit will be the Holy Grail to the long range cult!

 

[tuckner]

Re: Figuring FPS change as a function of temp

Would have been cool to see a temperature for that load around 50 degrees to nail down where the real swing is at.

 

[Sniper1*]

Re: Figuring FPS change as a function of temp

I'll see what I can do!

 

[hexades1]

Re: Figuring FPS change as a function of temp

I would guess the temp changes above the freezing point will be pretty linear. Below freezing point various powders behave very differently. Military powders typically tolerate cold pretty well because they have been specified to be tolerant.

Then if you choose to shoot more in winter you will se that speeds will go up as the barrel heats. When the rifle colls the spedd will do down given the ammo has been on constant temperature.

 

[Greg Langelius *]

Re: Figuring FPS change as a function of temp

For sustained firing, I have my doubts about calculations involving ambient temperatures and velocities.

Once the barrel heats up, a round spending much more than a few seconds in the chamber before firing is likely going to start showing velocity deviations from predictions based on ambient temperatures.

I think load testing and muzzle velocity readings should be done following customary barrel heating and at the sort of shooting cadence the intended application is most likely to share. Honestly, I think the muzzle velocity results will share very little with ambient conditions.

Greg

 

[rmfield]

Re: Figuring FPS change as a function of temp

Lots of good practical suggestions here, but I think the matter of muzzle velocity changes due to ambient temperature changes remains one of the least understood aspects of precision shooting ballistics. I have yet to see a well controlled study on the matter, but would welcome one. Even if you manage to get a handle on the temperatures of all of the compenents at the time of the shot, you still have to deal with the vageries of consumer chronographs.

 

[Goose375]

Re: Figuring FPS change as a function of temp

i saw a pretty neat demo once upon a time... a round was placed in an MRE heater and a round was placed in ice water... One was shot right after another at an E Type at 300m.. Both rounds hit the target... only about 4-5 inches apart... I would argue that any 1 degree change in temp is negligable. Maybe 10 degree increments would give you more real world results. Seems like you get your one time DOPE at any given temp... type in the MV into the calc and get your drop.. that would be repeatable enough for field work...

 

[Lindy]

Re: Figuring FPS change as a function of temp

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">I have yet to see a well controlled study on the matter, but would welcome one. </div></div>

I think you should get busy and do one. After all, the temperature is going to drop soon up there, which would make it perfect for cold temperature testing...

The problem, of course, is that it would have to be extensive, because you wouldn't want to test just one powder.

 

[rmfield]

Re: Figuring FPS change as a function of temp

Yeah, to do it right would be a daunting task.