Wind at 100yards

[ovenpaa]

I have noticed that some loads will move the POI to the right by as much as they move them up when doing load development.

Are you far enough away that the bullet has stabilised?

Last time I observed this was at 200m and was whilst incrementing loads by .3 grains from a base of 59.4 - the shots climbed high and right with my 7mm and I have seen .308 climb right as well.

[Dougan]

Good point Dangermouse...even more to consider :?

So it seems there is a vast amout of variables that will affect accuracy - some of which you may not be able to fully understand or account for. But if you want to make sure that you are on target right from 'shot 1', and stay on target - then maybe (apart from knowing how your rifle behaves as it warms up (gets fouled)) you need to forget adjusting from just one pre-set 'zero'...and have various 'zeros' recorded for different ammos and/or distances....?

[ovenpaa]

With no wind the zero for windage should remain constant however spin drift and coriolis effect come into play but this can be determined as a matter of a few inches at one mile.

EDIT - My spreadsheet for my load (155 Scenar @2900fps) at zero wind shows:

400y 0.3MOA

800y 0.8MOA

1200y 1.7MOA

1600y 3.0MOA

ANOTHER EDIT - Numbers are very similar for the 22C (80 grain .224 @3250fps) and 7mmC (180 grain .284" @3000 fps)

[Dougan]

With no wind the zero for windage should remain constant however spin drift and coriolis effect come into play but this can be determined as a matter of a few inches at one mile.

EDIT - My spreadsheet for my load (155 Scenar @2900fps) at zero wind shows:

400y 0.3MOA

800y 0.8MOA

1200y 1.7MOA

1600y 3.0MOA

ANOTHER EDIT - Numbers are very similar for the 22C (80 grain .224 @3250fps) and 7mmC (180 grain .284" @3000 fps)

Right I am confused now - when you're talking about MOA, are you talking about 'windage' - if so then does that not show that your 'zero' (as it is at say 200 yrds) will change with distance...0.8 MOA is more than 6" at 800yrds...worth accounting for...

Also when you said "Last time I observed this was at 200m and was whilst incrementing loads by .3 grains from a base of 59.4 - the shots climbed high and right with my 7mm and I have seen .308 climb right as well"....does that not show that the 'windage zero' (and not just elevation) changed with the changes in ammo....?

[ovenpaa]

Right I am confused now - when you're talking about MOA, are you talking about 'windage' - if so then does that not show that your 'zero' (as it is at say 200 yrds) will change with distance...0.8 MOA is more than 6" at 800yrds...worth accounting for...

Also when you said "Last time I observed this was at 200m and was whilst incrementing loads by .3 grains from a base of 59.4 - the shots climbed high and right with my 7mm and I have seen .308 climb right as well"....does that not show that the 'windage zero' (and not just elevation) changed with the changes in ammo....?

Ah, I think we are discussing slightly different things here. If you are shooting your .308TR in zero wind them my chart in the previous post is going to be reasonably close to your load in your rifle, in fact I was surprised at just how little it changes give differences in MV, BC and projectile weight. This mean a .308 RUAG versus a .308 Lapua verus a .308 PPV is going to show a slightly different 'windage' moment at say 600 yards, however it is not a huge difference, probably around .2MOA maximum which is under half of an F Class V Bull.

When I was testing the variable at 200m was speed of projectile (MV) the projectile was a constant and the odd thing is increasing the MV should only give a small change in point of impact, certainly less than .1MOA so I should be stringing the bullet holes in an extended clover leaf yet with some loads I see greater dispersion than this and always in a up/right trend

[Dougan]

Right I am confused now - when you're talking about MOA, are you talking about 'windage' - if so then does that not show that your 'zero' (as it is at say 200 yrds) will change with distance...0.8 MOA is more than 6" at 800yrds...worth accounting for...

Also when you said "Last time I observed this was at 200m and was whilst incrementing loads by .3 grains from a base of 59.4 - the shots climbed high and right with my 7mm and I have seen .308 climb right as well"....does that not show that the 'windage zero' (and not just elevation) changed with the changes in ammo....?

Ah, I think we are discussing slightly different things here. If you are shooting your .308TR in zero wind them my chart in the previous post is going to be reasonably close to your load in your rifle, in fact I was surprised at just how little it changes give differences in MV, BC and projectile weight. This mean a .308 RUAG versus a .308 Lapua verus a .308 PPV is going to show a slightly different 'windage' moment at say 600 yards, however it is not a huge difference, probably around .2MOA maximum which is under half of an F Class V Bull.

When I was testing the variable at 200m was speed of projectile (MV) the projectile was a constant and the odd thing is increasing the MV should only give a small change in point of impact, certainly less than .1MOA so I should be stringing the bullet holes in an extended clover leaf yet with some loads I see greater dispersion than this and always in a up/right trend

OK..so not much difference between different ammos - and therefore too small to worry about in TR ...thank god you get 2 sighters

About the load test you did at 200 yrds - and I may well be getting 'out of my depth' on this ( ) - I read somewhere else on the forum about bipods, and the 'torque' effect - Am I right in thinking that this can push bullets 'up and right' (on RH turns) due to the 'jump' of the barrel?...and is it possible that this effect could have been increasing as the power of the rounds was increasing...?

[ovenpaa]

OK..so not much difference between different ammos - and therefore too small to worry about in TR ...thank god you get 2 sighters

About the load test you did at 200 yrds - and I may well be getting 'out of my depth' on this ( ) - I read somewhere else on the forum about bipods, and the 'torque' effect - Am I right in thinking that this can push bullets 'up and right' (on RH turns) due to the 'jump' of the barrel?...and is it possible that this effect could have been increasing as the power of the rounds was increasing...?

I was shooting off an F Open rest so it should be tracking reasonably well and yes the torque effect will cause a narrow footprint bipod to hop which is why the F/TR bipods are nicer in this respect, the wider footprint offers increased stability reduced hop which in turn allows you to get back on target quickly.

The spin drift and coriolis effect has set me thinking, I have the numbers but have not paid much attention to them, especially the differences seen when compared to MV (Spin drift should be amplified as MV increases)I will spend a few minutes tonight looking at the differences with respect to projectile weight and speed, of secondary interest geographical location and direction of shot should be interesting as well.

I feel some math and spreadsheet coming on :?

[TattooedGun]

welcome my friends to my introduction to spin drift...

I had no idea this effect even existed, but yet again, it makes sense... the bullet is spinning why wouldnt that make it slide through the air (slightly) sideways!

coriolis effect, well sure, i got introduced to that from the film "shooter", much like alot of other people I expect...

[Dougan]

I feel some math and spreadsheet coming on :?

Yes bring on the maths :geek: .....I know Mike357 would particulary like to see it :lol:

I'm sure I read somewhere that on a modern naval gun (can remember which caliber), the targeting computer will 'aim off' 200 feet (!) at 14 miles, to allow for the coriolis effect.....?

[EagerNoSkill]

late comment :-)

The "coriolis effect" is only likely to be measurable at extreme ranges - like 2000 m - it is usually an artillery problem and it worsens as you near the equator and "spin drift" is also relatively small and not likely to show dramatically up at 100 / 200 / 300.

But could the " barrel harmonics" and too much lesser extent "lock time" impact.

I found in my 300Winmag that 180 g bullets from different manufactures (hunting loads for magaizine fit - so I had same OCL for all) could have marked differences. The dispersion was often sideways (nevermind the up or down)

So each bullet has a different Ogive, different bearing surfaces, engagement pattern with lands etc.

It all accumulates to different degrees of impact on the barrel harmonics and the exact flex point at moment of bullet / barrel release.

Similar things "may" happen when you change powder or primer - the bullet groups loosens or tightens up - same case & same head - but differrent burn rate / sequence.

Let me leave before I prove my ignorance even more!
:?: