Tail wind help needed

[Pester]

Shooting 100yds, 168grn fgmm .308 out of Rem 700 Bartlein/Manners. Bench with sandbags.

I was getting .5 inch groups down to all shots touching . All 5 round groups and light/no wind. As it warmed up the wind picked up and it was a pretty gusty wind but no idea exactly how strong it was. My groups grew up to 1" with most being .5 or a little bigger. What I noticed was that the groups all had about same left/right spread but the vertical jumped to almost double.

Would I really expect a tail wind to move my groups vertically like that over 100yds? I think I was getting lazy or something 'cause it does not seem like a tail wind would move it around that much. Right?

So if most of it was me what should I re examine? I thought I had the same build up and I went through my mental checklist but my groups really spread out. Cheek weld?

this seems so remedial but I could not get back down to my smaller groups. Makes me want to take up underwater basket weaving......

 

[KYpatriot]

Are you shooting over level ground? Objects, humps/hills combined with wind will create up or down drafts which can present as a "vertical crosswind" for lack of a better term.

 

[Pester]

Thanks KYPatriot,
Generally low rolling hills that run maybe 20-30' high. The area I was shooting in was a low box canyon with the surrounding hills being no more than 20' high. It is almost flat for a mile or two where there is a seasonal creek and then a slight incline up to where this low U shaped valley sits. This 'valley' floor is flat, like they had row crops there and flood irrigated, so my perception was that the wind came across this larger valley, and then funneled into this 'valley' and then up over these little ridges.

Calm through most of the morning and then the wind was coming in straigt at my back. Ballistically would a strong breeze push a 168grn .308 around like that over only 100yds? If I were to guess it was maybe a 20mph wind.

Not complaining....all shots touching is a good group for me at 100yds. Just trying to improve consistancy and cant figure out if I need to improve reading the wind or its me so any help is greatly appreciated.

 

[KYpatriot]

It only takes 7-8mph or so of crosswind to move the bullet a half inch even at 100 yards. It also takes very little error on the part of the shooter to move it that much, and high winds not only affect the bullet but can affect the stability of the rifle while firing as well. It is not uncommon to see your heartbeat in the reticle from prone, so gusty winds will easily show up in the crosshairs as well.

 

[damoncali]

As your groups get smaller, it gets exponentially harder. This is why savvy shooters take 1/4 MOA claims with a grain of salt. Shooting that small even at 100 yards requires above average wind reading skill or very calm conditions Throw in that wind gusts near the muzzle can do all kinds of weird things at short range, and it's a challenge. Don't rule out position (which is also important), but wind matters at 100 yards too when you're in the 1/2 MOA range or below. Check out all the wind flags at a short range benchrest match. It looks like a scene out of alice in wonderland with all those spinning daisy wheels.

 

[jim_k]

I doubt the tailwind did anything at 100 yards. The issue with a tailwind or headwind is, how much does the wind change your time of flight, in raw milliseconds. A full 30 mph tailwind is 44 FPS, but it isn't adding 44 fps to your bullet. The bullet simply uses its first 44 fps "catching up" to the wind. From that point on, it's just a matter of slightly reduced wind resistance. Do the ballistics as if the wind actually added 44 fps to the bullet for a 100 yard shot, and you'll see. At 100 yards, not much, as in not enough to see on the target.
Is your action bedded? If not, get it done. Then see what happens.
Jim

 

[Pester]

Right....the number of times I get 5 shots all overlapping only happens with calm winds and not always that often. The place I shoot tends to get pretty gusty in the afternoons so I have gotten used to my placement stringing out horiz. but what caught me by surprise was to see them run out vert. with that tail wind.

My suspicion is that I am driving this so I will go back to the drawing board. I already checked scope,rings, etc and all is tight. I do find it easy to not get into the right, relaxed position so I will also be more dedicated to building my position consitantly each time.

Looking at the weather I might be in a position to see the same conditions this Sat so I will get there early and also try seated and prone to see if there is a difference.

Thanks folks.

 

[jashmore1234]

Is the wind moving the shooter?

 

[KYpatriot]

Is the wind moving the shooter?

Probably. Unless there are terrain features creating ramps for the tailwind that convert it to vertical crosswinds. Or both are happening. Good that the OP is open minded about it, because he is, likely he will figure it out.

Recently over on another site, long range hunting, I suggested to an experienced shooter there that the fact he was getting smaller moa groups at range than at 100 yards was probably not the bullets going to sleep but a shooter effect, and it was clearly viewed as an insult. I have little patience for people who attach their identities or sense of self worth to a mechanical device like a rifle.

 

[crowdog7]

Is the wind moving the shooter?

This was my first thought, especially with an estimated 20mph wind.

 

[Pester]

Probably. Unless there are terrain features creating ramps for the tailwind that convert it to vertical crosswinds. Or both are happening. Good that the OP is open minded about it, because he is, likely he will figure it out.

Recently over on another site, long range hunting, I suggested to an experienced shooter there that the fact he was getting smaller moa groups at range than at 100 yards was probably not the bullets going to sleep but a shooter effect, and it was clearly viewed as an insult. I have little patience for people who attach their identities or sense of self worth to a mechanical device like a rifle.

I dont have a ton of ego in this unless wanting to get better is considered ego.

Never thought about the wind moving me around so I will check it this Sat if the conditions are similar. I do recall the wind gusting and taking a bit longer between shots to settle so maybe thats exactly it. I am not knowledgable enough about wind dynamics to know how a vertical wind would occur. The lane I shoot has a larger open field(20+ acres) that funnels into little alluvial plains with the one I shoot at being flat for about a 100x100yd space and about a 3' rise over the 100 feet I shoot.

Thanks for the input guys....appreciated.