Predicting POI based on temperature?

[Capt. Charlie]

I'm mostly into defensive handgun shooting and relatively new to centerfire precision rifle, and I know that altitude, humidity, and temperature can affect a bullet's point of impact, but I was utterly amazed at how much effect temperature can have.

Earlier this summer, I finally got around to sighting in my Remington 700 BDL chambered in .270 Win. The scope is a Nikon 3X9X40, and for now, I'm zeroing at 100 yds. This rifle seems to prefer Federal's Vital Shok, 130 grain ammo.

When I sighted in, the temperature was approx. 80 degrees F., and I ended putting 5 rounds in a 1.5" pattern (benched), exactly where I wanted 'em.

Afterward, the rifle was put away into a hard shell case, and I'm sure it wasn't bumped or otherwise disturbed.

A few days ago, I took it out for another shoot, with everything being the same, except for the temperature. This time, it was in the high 30's. Windage was right on, and I had the same, tight group, but this time it was a full 6" low!

After a bunch of cussin' & fussin', I've finally concluded that the temperature drop was the culprit, and that I'll have to compensate for that in the future.

Now, my question is, how do I accurately compensate? Are there tables out there somewhere that list the drop at specific temperatures for specific calibers and bullet weights? Is there a formula to calculate drop?

Your input appreciated.

 

[arkie2]

There's a good discussion at this link. If the Federal rounds you're using have temp sensitive powder that could be part or all of the problem. The only formula I know of, especially if you're a hunter, is to fire the rifle before you use it in conditions you think you'll encounter on the hunt. There's some good discussion there on the effect of temperature on hot loads which were loaded in cold weather and fired when it's hot.

http://www.gatago.com/rec/guns/25631839.html

 

[Jimro]

heat and velocity

We were taught that 20 degrees farenheight corresponds to a 50 fps difference in 5.56 ammo.

This means that your 50 degree difference would have had a 125 fps difference if the same ratio holds true for your 270. Since the 5.56x45 and 270 both operate within the same pressure range it's not and invalid assumption.

The difference in velocity also means a difference in recoil and internal ballistics. A rifle can move up to 1/16 of an inch before the bullet clears the muzzle. What it sounds like to me is that the slower the bullet moves the less it kicks the muzzle up before it leaves, causing it to print lower than normal.

This "kick the muzzle up" effect is often seen when folks who normally shoot 150 grain bullets for deer switch loads to 180 or 200 grain pills for elk. Oftentimes the heavier bullets print higher than the lighter bullets because of the muzzle rise due to internal ballistics.

Jimro

 

[Buckythebrewer]

You should see the shift @ 600+ yrds in P.O.I. I have seen a big shift(probably about 1ft +/- )Just from the sun rising and changing the temp of my rifle and scope(actually normal)..Here in Maine we can have a cold early morning sometimes and then as the sun rises closer to noon the temp changes quite a bit and it had quite an effect on the metal tolerance.1/8 click at 100yrds turns into alot more at 600 yards(Im talking windage more than elevation).I never worry about zeroing my scope.I don't use it for hunting so I just use kentucky windage at longer ranges(as long as its somewhat near anyways).I just take a couple sighter shots and adjust to something to aim on.I do the same at 100 yrds.It is actually better to have your scope Off because If your scope is zeroed your shots will shoot out the point your aiming at (at least with HIGH X scopes like 32x and up anyways) ..So for hunting I would take Jimro's +Arkie2's advice,But for target shooting and load changes I would just not worry about it until you wan't to place that bullet on an Animal or exact spot like competition shooting or whatever..Just get a point to aim at and let em group.JMO

 

[30Cal]

Increase in air density is the biggest driver for POI shift, but ball powders will also vary quite a bit with temp, so as the temp drops, so will velocity at the same time, air density is going up causing the bullet to drop a lot more.

I use 20ºF = 1MoA as a general rule.

 

[ZeroJunk]

You guys have discussed this at an opportune time for me.I sighted my rifle in at 80 degrees.I am going to Montana on Monday and will re-zero at probably 40 degrees.Will be interesting.May get close to zero in the mountains there next week.

 

[joshua]

I have never seen my POI change that much especially with the 270Win. Just wondering if you used the same rifle rest, shooting jacket and so on. The combination of different pressure of the ammo, air density and a drastic change in how the rifle is rested/held could provide a drastic change in POI. I've seen that much change in POI just by switching from sandbags to bipod. The bipod puts upward pressure on the stock at the front swivel while when I use sandbags the upward pressure is closer to the action. During the hot weather you probably weren't wearing a jacket, while when it got chilly you put on a medium to thick coat that is made of slippery polyester with some insulation material between your rifle's buttstock and shoulder. josh

 

[Ammo Junky]

6" at 100yd was not from temp change. Did you remove the action from the stock? I can think of a lot of things that would change zero, but none that would be 6". Big temp change, or humidity, different hold / pressuar on forearm, different ammo / lot# of ammo. Make sure your action screws, scope mounts and rings are tight. Not likely the problem with good groups, but you never know. Problems have a way of working in strange and unlikely ways. Let us know what you find.

 

[Buckythebrewer]

Using the same ammo on the same day,I have seen +/- 2"-4" differance in POI @ 100yrds just from changing the front rest location(using sandbags) on my 24" ar15's floating tube(shifting from near the mag ,to half way,and then out to the end).
IT probably wouldn't have been as bad if I had more baby powder on the sandbags(couldn't find the damn stuff)..
Alot of factors can change POI.Most of the time its something you forget to remember throwing you off.


Zerojunk,,You might want to bring a sweater