temperature effect

[bman73]

I went back out to the range the other day and was shooting at 1150 again, did mangage to get some gong hits on camera, however I had a serious case of plumbers butt going on and am not willing to plaster that image over then net. I did however make an observation that kind of has me perplexed. The last time I was shooting the temperature was -8C and yesterday it was +6C, with a temp swing of 15 degrees I noticed no difference in my vertical dope, the wind was in completly the opposite direction and stronger as well as gusting compared to the last time. Would any vertical shift have been masked by the gusting winds at that range or does temp need more of a swing to make a difference in POI? On the plus side my grouping was much tighter this time, my wind reading does need more help though as that cold bore first shot hit still eludes me. Back to the .22 at 200 yards for more wind practice I guess. Thanks in advance for any answers

I guess I should mention what I'm shooting unless everyone who reads this is a mind reader huh? lol
.338 LM Savage 110 BA
300 grn berger OTM
91 grn H 1000 Fed 215 primer Lapua brass

 

[aroshtr]

Crosswind will effect your vertical impact. How much is the tough part. For a Right to Left wind you will hit higher, and a Left to Right wind you will hit lower. From my basic understanding it is the same effect as spin drift. In a no wind condition the right twist spinning bullet is falling through air which has higher pressure on the bottom of the bullet and it tracks right slightly like a spinning tire (Spin Drift). Same think happens in the wind when there is more pressure on one side of the bullet. I know Bryan Litz has a calculation for this in his book, but I don't have it, and probably could not make sense of the calculation. I would take a wild *** guess that in a 10mph wind the vertical drift might be close to the amount of spin drift your bullet would experience. Maybe someone who understands this better could chime in with some real calculations.

 

[Broz]

I just ran the numbers in my Applied ballistic program for a 1150 yard shot. The dial up elevation correction never changed at all from a 10 mph 9:00 wind to a 10 mph 3:00 wind.

Now for temperature the difference in a shot at 25 degrees F. called for 29.6 moa. up. and for 37 degrees F it called for 29.3 moa. up. So you would see a difference of .3 moa at 1150 yards at my altitude of 4100 ft. asl. That would be about 3.6 inches.

Jeff

EDIT: I re-ran these numbers for my 338 LM with a 300 Berger and a MV of 2800 fps.

 

[aroshtr]

None of my programs seem to calculate it either, but I have a benchrest friend that told me about it years ago, and found some post on another forum. I wish I had his book, but maybe somebody will have a copy. Some of the excerpts from the other forum are below. Maybe this info is flawed, but it makes logical sense that it would have some effect.

"Later on I looked up my Bryan Litz ballistics book and found a (complex) formula to predict the vertical displacement fairly reliably (P78 or thereabouts) using the Gyro Stability factor of the bullet being used along with its length and calibre plus other factors....... and the calculations do work out close to field-based reality...... the man is a God!"

"I've just been sad enough to work all this out using Bryan's formulae - for your 1:8 3000fps 0.264 123grscenar at 200 yards 5mph left wind would require an additional 0.2MOA to be applied so about one and a half 1/8MOA clicks and a 16mph right wind would require 0.6MOA to be taken off so about five 1/8MOA clicks"

 

[bigngreen]

I've only shot once with high enough wind and range to see Applied Ballistics give me a correction and it was only a few tenths of an MOA at 1980 yards with a 17mph cross wind, but it was the first time I realized some of these programs are taking wind direction into account but you have to be maxing out your conditions.

 

[Broz]

The numbers I ran for the OP were from my 338 Lapua with a 300 Berger at the distance the OP listed, 1150 yards. I felt this was useful as the combination I use has 1400 plus long range shots through it and it is a proven cold bore contender. The corrections I listed are indeed from Bryan Litz new Applied Ballistics program.

Jeff

 

[bman73]

So I guess a temp swing of that amount really doesnt have much effect on trajectory at all then at these ranges, or is it more a case of the bullet I'm using just being that efficiant and not really caring all that much about really anything that gets thrown at it. As a side note I was out again this weekend and just for ***** and giggles tried shooting my 270 winny at 1150 as well as my dad trying his 7 x 57 mauser with peep sights. Not nearly enough adjustment on either to get there so we just tried holding over, Lets just say we'll be sticking with the Lapua at that didstance. Dad however was hitting the 4" x 9" gong at 420 yards with his 7 mauser. We tried to get it on film but the camera we had didn't really pick up too much. Was still pretty proud of the old fart though

 

[Broz]

So I guess a temp swing of that amount really doesnt have much effect on trajectory at all then at these ranges, or is it more a case of the bullet I'm using just being that efficiant and not really caring all that much about really anything that gets thrown at it.

Well, not too fast here. I just noticed I used Fahrenheit, you used Celsius. We both used 15* difference but I have a feeling that your 15*C is a lot larger difference than my 15*F. My app does not do C so I am not sure we have an accurate answer yet.

Jeff

 

[HARPERC]

According to an online calculator -8c = 17.6f, 6c = 42.8f. A difference of 25.2f

 

[NZ Longranger]

You definitely should have noticed a difference in POI with a 14 degree Celsius temp change, about 7 or 8 inches according to my calculations. If you noticed none, then something else was at play here.
Aerodynamic jump is the spin related vertical component of a crosswind and we've seen it on many occasions and always allow for it beyond 1000 yards or in strong winds. Bryan covers it well in the chapter on wind deflection in his second book, but his AB program doesn't allow for it unfortunately, so you have to do a simple fudge factor.
The simple allowance we worked out from Bryan's info is .3 MOA for every 10mph of full cross wind, taking it off for a 3 oclock and putting it on for a 9 oclock. It seems to be approx the same for all calibers and bullets. Maybe Bryan can come on and verify if we've got this about right? It certainly seems to work for us.

 

[Broz]

According to an online calculator -8c = 17.6f, 6c = 42.8f. A difference of 25.2f

Great Thanks! So lets run these numbers again using those corrections.

338 LM / 300 gr Berger @ 2800 fps. ALS 4100 ft.

17.6* F dial up = 29.5 MOA
42.8* F dial up = 29. MOA

a difference of .5 MOA or about 6 inches difference in point of impact.

Jeff

 

[NZ Longranger]

Jeff you're right. I originally ran the numbers at sea level for 7.5" as I couldn't remember what altitude Bman was shooting at. Just ran them again at 4000' and got a 5.7" difference. Almost 2" difference between seal level and 4000' for the same 14 degree Celsius temp change - interesting!
Greg

 

[sendero72]

I was hunting last weekend and the shot would have been just over 1000ys. I ran the Sierra and the G7 program which gave me a .25 moa change for every 10 degrees change in temp. I did not try to factor any wind in the program.

 

[bman73]

I guess I can chalk up my not noticing much of a difference to my level of inexperience at these ranges then. I am shooting at roughly 2400 feet above sea level by the way, forgot to mention that previously, sorry. I have noticed that I am getting better at this but I am thinking that in order to get to the next level as far as my abilities go I am going to have to start using a rear bag instead of just my fist under the stock. It works great at shorter ranges but not as consistent as I want/need to be at extended stuff. Thanks to all for answers posted on this. Will definately being paying more attention to temp changes from session to session and keeping a better log of corrections for temperature made at the range