what does the weather do to a rifle.

Discussion in 'The Basics, Starting Out' started by bigben, Nov 28, 2004.

1. bigbenActive Member

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Nov 17, 2004
What does the weather have to do with rifles and how they work. also what kind of yardage are we talking about in how far out the weather makes a difference. Also what kind of rifle will a .224 caliber be different then a big mag bullet with the weather.

2. cdmortenWell-Known Member

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As a general rule, for every 20 degree drop in temp, you need to "come up" 1 MOA. The thought being that as the temp declines, you get closer to the dew point and hence the humidity rises which causes more friction on the bullet.

3. bigbenActive Member

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what distance does this start taking affect on. I am guessing a 1/4" at 100yds won't be the same as 300 yds. Basically if I can get it within a 3" circles at 300 I will be happy.

4. BrentWell-Known Member

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1/4 MOA =

1/4" at 100
1/2" at 200
3/4" at 300
1.0" at 400 and so on.

At what distance it becomes a problem is determined by the size of the target, the rifles capability, the shooters capability with the rest he has and the weather conditions he is currently dealing with.

Example:
The rifle is capable of keeping every shot within .5 MOA of point of aim (POA), or all shots within a 1.0 MOA circle centered on your POA.

Now enter a little shooter induced error and you may now only be able to keep all shots within .75 MOA of POA, or in a 1.5 MOA circle.

Temperature has increased 30 deg F. at mid-day, up from 60 deg to 90 deg, and now changes your POI from POA, say .75 MOA.

At the same time a low pressure system has moved in and thus atmospheric pressure has dropped from, say 29.50 to 28.50 hg in the last 6 hours. This may mean about .25 MOA.

If your ammo is not in your chest pocket or near your body some other place and at the same temperature you sighted in with, you have one more variable to contend with... a MV change. We will say it is a non issue and remains consistant.

Your target is at 300 yards.

We know the rifle is capable of 1 MOA at 300 yards, or 3" groups consistantly.

We know the shooter is only capable of holding groups consistantly inside of 1.5 MOA at 300 yards, or 4.5" with this previously tested field rest at 60 deg F. and a 29.50 BP in zero wind, his logged sight in conditions.

If he holds, or dials 3 MOA up elevation for the new 300 yard zero, POI will be .75 MOA high, or 2.25" as a result of the 30 deg increase in air temp.

Air density has also decreased due to the reduction in atmospheric pressure and POI will be high by .25 MOA, or .75" at 300 yards.

You subtract .75 MOA elevation for the air temp change, now at 2.25 MOA.

You subtract .25 MOA elevation for the BP change, now at 2.0 MOA.

POI would have been roughly 1.0 MOA higher, or 3.0" than expected if these two variables were not accounted for. Remember, this is with a stable and predictable MV unaffected by the temp change.

If the same shot was to be taken at 600 yards, that same 1 MOA error, if not accounted for, would have meant a POI shift of 6.0".

Now the question remains; What is your target's size and what POI to POA deviation is acceptable. Also, and just as important, at 600 yards, am I still able to keep all of my shots consistantly within a 1.5 MOA circle and even more importantly, each shot within .75 MOA of my POA.

The fact of life is your ability to maintain that will without a doubt degrade as range is increased. Learn what you can maintain at various ranges, be honest with yourself about it and you will have a solid baseline from which to work from and progress.

Good luck.

5. BrentWell-Known Member

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If it's just air temperature that changes, I've found about 1/4 MOA lower POI for every 10 deg F. drop in temp, that's just what I've found... Of course, if the barometric pressure drops at the same time, it may be enough to cancel out part, or all of the effect of the temp drop. Air temp and BP both alter the density of air, this is what is altering your trajectory. Your ammo temperature dropping can also reduce MV and then it will also contribute to a POI shift.

6. BrentWell-Known Member

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A change in BP of 1.00 will generally cause around 1/4 MOA POI shift in my experience.

Bullets with a lower BC than .6 - .7 may be effected more, I just haven't spent much time testing them. Most ballistic programs will show you these effects.

MV and BC alone determine trajectory.
A bullet leaving the muzzle going a little slower, but having a higher BC than another will generally drift much less in the wind, even though the trajectory may be identical, or very close... it will also retain more MV and thus more energy at long range.

Choose a high BC bullet in any caliber you shoot for real LR work, a cartridge case large enough that it will push the bullet to the desired MV, and a barrel with a twist rate fast enough that it will stabilize these much longer projectiles.

7. MysticplayerWell-Known Member

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Jul 27, 2001
big ben, I consider changes in weather to involve changes in humidity and temperature. I will not discuss climate and elevation as that has to be dealt with in your drop table.

Changes in ambient humidity can have drastic affects on the stock. This will change POI and rifle tuning. Solution is to use synthetics or lam. stocks.

With temp., the vel of your load may change. That depends on how much temp difference you are talking about. I use the Hodgdon Extreme powders to avoid this problem and it has worked well for me from 100F to freezing.

Besides a change in vel, the load may also change pressures which can affect accuracy enough to cause serious problems.

Temp will also affect your stock and when really cold, your barrel. The way a barrel vibrates can change dramatically when it gets very cold.

The only way to find out is to test under the conditions you plan to hunt. That is not always possible so at least try and let the rifle settle down a day or two at your new destination. Do some sighting in just to make sure all is well.

Jerry