temp. effects on bullet velocity and drop

[mrbungle]

I went shooting this morning at 533 yards. My book says 3.6 mils from a 200 yard zero. Well, first two rounds were a misses. I could see i was shooting high so I dialed the scope down to 3.3 mils. bang every time from prone and even leaning over a truck bed 3 out 4 shots.

i have changed loads recently but not since i was shooting at this range the last few times.
the load i am running is 45 grns of varget with a 165 grain bullet in .308. I do not know the velocity yet.

The temperature today was in the low 70's and it has been in the high 80's when I have been shooting. I live in alabama so the humidity was high as always.

So is there a way i can count for these temperature drops?

 

[40xs]

I use varget and it has very little varience due to heat differential, and when it does it's in the opposite direction; hotter=higher elevation, colder= lower elevation. Varget is one of the better powders to use for temperature change. I can not figure why you would experience the change you saw. the outside temps. are the ones we are figuring on. Where was the ammo stored? were they stored in a warm area? if so, they may not have had time to acclimate.

 

[usmc02xx]

I went shooting this morning at 533 yards. My book says 3.6 mils from a 200 yard zero. Well, first two rounds were a misses. I could see i was shooting high so I dialed the scope down to 3.3 mils. bang every time from prone and even leaning over a truck bed 3 out 4 shots.

i have changed loads recently but not since i was shooting at this range the last few times.
the load i am running is 45 grns of varget with a 165 grain bullet in .308. I do not know the velocity yet.

The temperature today was in the low 70's and it has been in the high 80's when I have been shooting. I live in alabama so the humidity was high as always.

So is there a way i can count for these temperature drops?

is your data book thats giving you 3.6 mils built off of actual shooting data, or a ballistics program with a guestimated velocity? Im assuming real life hits, but you mentioned not having chrono data like it could be important. I've never seen alot of variance in varget either, but i have seen wildly innacurate ballistic charts due to wrong velocity.

 

[mrbungle]

yes, the 3.6 in my book is from actual shooting data at that range.

 

[mrbungle]

zero was good, i was working on 200 yard groupings before shooting the 533.


it has to be the load. i did something different. i guess.

 

[nuclear_shooter]

I'm no expert, but as a physicist I can tell you that hotter air will have less drag on a bullet than colder air. I believe JBM and other calculators are definitely capable of taking this into account. Also humidity and the like will have an effect. Will it be 3 mils? I don't know... but you can play around with the ballistic calculators and see. Also the rate at which your powder burns will depend on temperature. Varget is one of the 'Extreme' powders which is supposedly not very temp sensitive and be fairly consistent lot to lot. I also shoot an Extreme powder (H1000) and am very curious to see how this holds up. You can read more about the Extreme powders here: http://www.hodgdon.com/smokeless/extreme/page2.php. Remember that this is a marketing tool, and we all know how lab tests vary from field results.

Edit: Just to clarify, there can be a MV effect from temperature as well as an external ballistics effect. You can measure and correct for the first, and JBM should be able to account for the second.

 

[mrbungle]

thanks, i am beginning to think it is a tracking issue with the scope. i am currently buying a new and much better scope than i am running now. i am running a millet 56 MM MIL/MIL scope. that i have put about 500 rounds through. i will find out next sat when i go to a precision rifle 101 class. we will have targets from 200 yards to 720, i believe.

 

[varmint slayer]

millet 56 MM MIL/MIL scope.

Problem found.

 

[mrbungle]

LOL

it was a great beginner scope. but like all things, needs change.

 

[nuclear_shooter]

LOL

it was a great beginner scope. but like all things, needs change.

If you're on a tight budget, you can get the SWFA SS 10x fixed power for $300. Its pretty great for the money, I'm very happy.

 

[mrbungle]

working on getting a us optics.