TAILWINDS effect on group sizes????

Discussion in 'Reloading' started by kraky, Mar 29, 2007.

  1. kraky

    kraky Well-Known Member

    Sep 4, 2001
    It's spring up here in Wisconsin which means weather from 30 degrees to 70 degrees but often winds of at least 10-15 mph per day.

    I went shooting the other day with a gun that has had a problem with vertical stringing. I found a load that works really good at 300 yds.

    Usually with sidewinds I get 4" horizontal groups. With no wind I get some pretty nice groups around 3" at 300 yds.

    The other day I'm shooting at 300YDS with 10-15 mph winds directly at the rear. I'm shooting out of a shed so maybe some wind comes over the top for a few yards. Anyhow I was getting vertical dispersion again....of about 5". Horizontal was about 2".

    Could the wind cause this??? I'm talking 180 tsx out of the 300 wby at about 3220 fps.

  2. goodgrouper

    goodgrouper Well-Known Member

    Sep 3, 2004
    A level tail wind will usually push the bullet up and then if it lets up, the bullet will go down and give you a vertical group. However, if you are shooting from under a roof, the wind accelerating over it can actually push a bullet down--hard. Up at my local range which sits at the mouth of a big canyon, the wind blows up the canyon after 10:00 am and the covered firing line faces slightly up the canyon so we get big time tail wind from 6 o'clock position or 5 o'clock. This wind races up over our slanted roof and once it gets to the other side it drops down from being accelerated exactly like a planes wing. This gives horrific vertical and the wind flags don't always show it because it is coming down at too steep of an angle. You just have to know it is there. One guy from Pheonix (I think) who was up here for a benchrest match took a smoke bomb, lit it, stood up on the corner bench and held the bomb up above the roof. You should have seen the smoke peel off the bomb and vertically circle down to the ground!

    And remember, wind effects everything from an 18 wheeler to a jet liner to a 180 tx bullet. It will effect that bullet even more if it is slightly yawing when the hardest vertical wind hits which would be about 20 yards from the muzzle. In other words, all bullets yaw when exiting the muzzle but the longer the bullet, the longer it takes to quit yawing and it will be very sensitive to vertical wind during this period.

    Try shooting out of the shed and away from anything behind you that might cause the wind to push down and see if your vertical improves.

    You didn't mention problems with standard deviations in velocity so I assume your load is good and is only giving you vertical because of outside sources.

  3. eddybo

    eddybo Well-Known Member

    Jan 5, 2007
    I was shooting my 6.5x284 the other day with a new load. Shoot at 300 in my gravel pit that is pretty sheltered from the wind. Got groups that were at least as good as my ability, with little if any vertical. Rode around to the other side of my land to shoot 500 and 600. I left my wind meter at home but had at least a 20 mph gusting tailwind. Of the five shots fired I had a nice little one inch three shoot group, with the other two nearly touching about 4 inches high. I like to think it was the tail wind, but it was probably just my shooting. I read in an article about this on (6mmbr I think) where they proclaimed that the vertical caused by a tail/head wind would be very small. If I get a chance I will find the article and post a link....but right now I am off to the gun smiths.