F T/R Competition Change in temp, change in dope

[XTR]

Just looking at my notes from shooting lately. I hadn't really noticed it because I've been leaving my 1000 yd zero on but with the temps in the mid 90s my 1000 zero has come down about 0.5 moa. I didn't record the atmospherics at the line this week but my last shot was an X and my elevation is actually down .625 from where it was 3 weeks ago. Now I wish I had recorded it.

I know that the ballistics programs calculate it, but it's interesting to see from the line.

 

[rwest309]

Re: Change in temp, change in dope

Happens big time to me in capitan. Usually if I shoot the first relay it's low 50s then by the last string of fire it's 90 plus. Makes those two sighters really important.

 

[KNIGHT11B4]

Re: Change in temp, change in dope

Yet another reason to read up on DA and understand it.

 

[ryhag1]

Re: Change in temp, change in dope

D Altitude is huge when the temperature changes that much over a day. Paying attention to detail is everything.

 

[XTR]

Re: Change in temp, change in dope

It's less of a big deal with the matches I've been shooting with unlimited sighters, but one of the matched this weekend we're doing a Full Bore with 15 for record, unlimited sighters at 800 then only 2 at 900 and 1000.

I think the 3x 1000 matches are all unlimited 20 rounds for record.

 

[notquiteright]

Re: Change in temp, change in dope

If your zero summary chart, come-up table, has columns for a range of temps you can get a handle quite quickly on how temp affects your strike of round.

Powder selection does alot to lower the temp spread. Is why Varget is so popular even though some think it is dirty.

Personally I've never seen as big a variation from 0 to 100+ with varget as with older standbys. I wouldnt consider a .5 difference much at 1000, I can have worse from strong winds.

Have fun exploring, just remember the recording

 

[XTR]

Re: Change in temp, change in dope

I'll have lots more opportunities, but the last couple of times I've shot I haven't pulled out my Kestrel. Lesson learned.

 

[Greg Langelius *]

Re: Change in temp, change in dope

I also have left my last 1Kyd dope on between F Class events. I use it as a starting point for sighter shots. I find that I adjust away, and then very nearly back as the barrel warms up. It's always a little different, but close, once the final dope gets established.

Greg

 

[kentactic]

Re: Change in temp, change in dope

do you think this more to do with temperature or other atmospheric conditions with temp just being an irrelivant peice of the puzzle?

 

[XTR]

Re: Change in temp, change in dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: kentactic</div><div class="ubbcode-body">do you think this more to do with temperature or other atmospheric conditions with temp just being an irrelivant peice of the puzzle? </div></div>

Mostly temp here. As noted above temp has a very big impact on DA. Humidity here doesn't vary that much and neither does barometric pressure, and while I do burn Varget specifically for it's temp stability, getting up into the 90s does change the burn rage some.

 

[Lindy]

Re: Change in temp, change in dope

Muzzle velocity is also a function of temperature. A really good powder/primer combination will give you 1 fps/degree F. of muzzle velocity change. So, if you chronographed your load at 70, and it's now 90 degrees, the round is going out the muzzle at 20 fps faster.

And I've seen powders with temperature sensitivity considerable higher than that.

At a grand, that makes a lot of difference.

 

[kentactic]

Re: Change in temp, change in dope

if temp affects the powder so much then why dosent barrel temp affect the velocity aswell?... and youd think after a couple shots its easily over 90 degrees inside the bore anyways even on a cooler day so the powder wouldnt know the difference.

bassically my question is why is it that you can heat up the barrel on a given temperature day and see no shift in POI but if its warmer out that WILL affect POI. somethings not adding up for me.

 

[notquiteright]

Re: Change in temp, change in dope

Leave the round in the barrel for a minute after running a fast 15. You will see a shift in the POI.

 

[XTR]

Re: Change in temp, change in dope

I never close the bolt and bake the round. Once the target is up, look at the wind, close the bolt, sight picture and fire.

 

[kentactic]

Re: Change in temp, change in dope

so then its the temperature of the air inside the case/powder temp, not the external temps that affect velocity? so when its hot out the cases heat up and affect velocity? so keeping them out of direct sunlight would be a good idea it sounds like.

 

[memilanuk]

Re: Change in temp, change in dope

Its a little bit of both... warmer air temps changes the density (lower) of the air between the muzzle and the target, affecting the drop (exterior ballistics). A round that picks up heat before firing, whether from baking in the sun or cooking in a hot chamber too long will have a more energetic ignition and potentially higher chamber pressures = higher muzzle velocity (interior ballistics). The former you can't do much about but you can compensate for it i.e. check the ballistics output of your favorite ballistics program for changes in temp from say, 40F thru 100F in 20F increments. The latter you can mitigate somewhat thru the use of (relatively) temperature insensitive powder/primer combinations, *not* leaving your ammo laying out in the sun, and *not* leaving a round to cook in the chamber while you dither over the wind

You can also go the extra mile and document MVs for your standard load at different ambient temps, ranging from cold bore in the freezing winter to at the end of a long string in the summer heat.

Me, I draw the line at that and go for the (relatively) temp insensitive load, bring a Kestrel and my ipod w/ KAC Bullet Flight and call it close enough...

 

[Lindy]

Re: Change in temp, change in dope

<div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">...why dosent barrel temp affect the velocity aswell?[sic]</div></div>

It does. Read this:

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

 

[kentactic]

Re: Change in temp, change in dope

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Lindy</div><div class="ubbcode-body"><div class="ubbcode-block"><div class="ubbcode-header">Quote:</div><div class="ubbcode-body">...why dosent barrel temp affect the velocity aswell?[sic]</div></div>

It does. Read this:

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

im unable to read that link Lindy.

 

[Lindy]

Re: Change in temp, change in dope

PM me with an email address, and I'll mail you a copy.

 

[PeterN]

Re: Change in temp, change in dope

I shoot two 308 loads at 600 and 800. I am at sea level and between 6oF and 90F factory Fed 168 GM moves 2.5 MOA at 800 but Varget loaded to the same velocity with the same Sierra 168 used in the Fed load only moves .50 MOA.