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Posted: 12/28/2003 9:05:36 PM EDT
At the risk of sounding stupid, I had what I thought was a strange problem today.  I've shot my two AR's, M1A and Shorty FAL at 100m before and knew them all to shoot anywhere from 2cm-3cm high at 100m because they all pretty much have standard military zeros.  Well today, perhaps by dumb luck, I found myself shooting into a 5mph headwind with gusts up to 10mph and ambient temps were droping about 1-degree every ten minutes with an initial temperature drop of about ten degrees when a cold front blew in just as we started setting up.

At anyrate, no matter what I did, no matter which gun I used, every shot fired landed AT LEAST, 8cm above point of aim.  Now, I've never fired these guns from a bench and perhaps that's the problem rather than wind or temperature.  Before today, I've never, ever, fired a rifle from a bench.  Either way, I couldn't seem to get the guns to settle down to a point that I was shooting anywhere near point of aim.  

I can at least say the windage was on.  Groups hovered between 3.5cm and 4cm on centers and were centered on the bull BUT, they were still 10-15cm above point of aim.  I screwed up the zero on one of the AR's so bad that I think it's now shooting about 20cm above point of aim at 100m and will need to be rezeroed at 25m before I can use it again.

Can anyone tell me if the temperatures and wind I saw today really had this much affect on bullet trajectories even at only 100m?
Link Posted: 12/28/2003 9:15:01 PM EDT
OFF hand, PUN Intended, I'd say that how you were resting the weapon at the bench and putting different stesses at points on each weapon, is the culprit, forend, bet ya money:)
Jack
Link Posted: 12/28/2003 9:24:06 PM EDT
I agree I might think temperature could be a factor but not the wind..not with .308 any way and wher you experienced this with a light and heavy caliber I would say diferent stress points or the temp drop.
Link Posted: 12/29/2003 3:20:07 AM EDT
At 100 meters, 10 mph wind and low temperature shouldn't make any differences to the POI.  You can start seeing the effect of temperature and altitude at 300 meters or so out, but not in close range.  My guess, these weapons are not free floated, and if you didn't zero the rifle from the bench, then I bet that's what caused your POI to shift.
Link Posted: 12/29/2003 3:32:14 AM EDT
AT least for the AR-

5-10 MPH winds will move the bullet(military 5.56 ball) 1-2 inches at 100 mts. per FM3-22.9
Link Posted: 12/29/2003 5:06:39 AM EDT
[Last Edit: 12/29/2003 5:51:09 AM EDT by gcmj45acp]
So it sounds like I MIGHT have been seeing the combined effects of different stress points caused by shooting from a bench versus using a sling as I did when the rifles were zeroed as well as possibly the wind and temperature factors.  But the stress points issue bothers me.  

I've fired from a barricade supported, off hand position as well as sling supported, sitting positions and a host of less than optimal conditions.  So what I don't get is what was a problem yesterday that hasn't been a problem before.

It might have helped if I'd had my buddy send a few downrange with the rifles to see if he had the same problem.  I'd had a new shooter start out on one of the AR's and his consistently high shot groups is what started me to moving the sights after I had the same problem when I tried shooting the same rifle.  That's when I noticed that it was happening with all of them.
Link Posted: 12/29/2003 7:46:23 AM EDT
A CROSSWIND of a few mph would shift POI at 100 meters. A HEADWIND might cause POI to DROP, a half centimeter at most. What you saw was 99.9% due to upward pressure on the forend.
Link Posted: 12/29/2003 7:59:49 AM EDT
I concur with Jack.

A headwind and low/falling tempertures cause bullets to drop FASTER.  So if anything they should have been lower.

Using a sling when zeroing will pull the barrel down.  So you didn't use the sling that raises the barrel, add the pressure from the bench pushing upward a bit more and you'll get high rounds.

FYI my FAL (.308) would shoot something like a 6" difference at 200y switching between using a sling and using the bipod.
Link Posted: 12/29/2003 8:19:46 AM EDT
Thanks guys.  it does sound like my basic problem is the fact that I was shooting from a bench rest and causing upward pressure on the forend and barrel.  I don't have freefloating barrels on any of these rifles(starting to miss the HK91 now) so that's definetly a probable cause that was only made worse by my lack of skill.  Hopefully I'll have time to work with the rifles again this weekend and I can find a place that I won't be forced to shoot from a bench.


 
Link Posted: 12/29/2003 8:54:32 AM EDT
I had a similar problem last winter with a .308 freefloated barrel from the bench. It was very cold -10 below I think at least.

My 100 yd groups were fine, but I couldn't hit a paper plate at 250 yds, when normally I do just fine. A guy next to me had an M1A and he was having fits too. He couldn't hit anything either. I just packed up and tried to forget about.

If I remember correctly my rifle and the M1A were hitting high, that's why I was so confused. Cold temp should lower velocity and drop your groups.

Really makes me wonder what happens to bullets at really low temps like that.
Link Posted: 12/29/2003 1:24:27 PM EDT
Lower velocity does not always produce a lower point of impact. In handload work-ups, barrel vibrations sometimes produce changes in POI that are not concurrent with changes in velocity.

Other factors, such as sling tension and and bench support stresses are the most likely. Sighting deviations also occur with changes of positions due to small changes in the relationship of the eye and the rear sight. If the wx was different than you had been shooting in, then the light may have been a factor.

On the other hand, it may be just a mystery, transient and unreproducable.  This is the art part of the science of shooting.
Link Posted: 12/29/2003 3:58:37 PM EDT
Cold temperatures will make the POI drop - cold air is dense, and creates slightly more friction upon the bullet as it travels downrange, therby reducing the velocity quicker. The general rule ( at least for .308s ) is that for every 20 degree change in temperature, the POI will rise/fall 1 MOA.

Considering that you also had a headwind, this would make the bullet drop another few inches - when you said that they were striking HIGH with these two factors, I thought I was going drunk ( yeah, bad fermentation and all that good stuff ).

Now that you mention the lack of free-floaters, that explains the problem altogether. One of these days, free float one of em, and I bet you those rules I mentioned will be true.
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