Coriolis effect

[6.5shooter]

Hi folks,

Something that I have wondered about how important or unimportant this is over long shots.

 

I know it matters but how much? Over what distance does it become a factor if all other things are correct like wind reading etc?

 

It must be more pronounced to slower rounds like 308 to say a 6.5x284 over distance because the bullet has more time in the air before it reaches its target?

 

The direction you are shooting? Do you use map references or a compas to determine if you are shooting north or south? Im guessing if you shoot east or west it wont matter?

 

Does anyone have any experience in this to answer my questions or am I just over thinking?

 

Garry

[DaveT]

Hi folks,

Something that I have wondered about how important or unimportant this is over long shots.

 

I know it matters but how much? Over what distance does it become a factor if all other things are correct like wind reading etc?

 

It must be more pronounced to slower rounds like 308 to say a 6.5x284 over distance because the bullet has more time in the air before it reaches its target?

 

The direction you are shooting? Do you use map references or a compas to determine if you are shooting north or south? Im guessing if you shoot east or west it wont matter?

 

Does anyone have any experience in this to answer my questions or am I just over thinking?

 

Garry

 

 

Garry the explanation is VERY lengthy BUT the difference at 1000 yards is minor (maybe 3 inches horizontal deflection (lost in wind-call and target shooters will compensate for this with 'sighters' anyway) whilst it is irrelevant to hunting distances.

 

Only really relevant for '1 shot kill' military snipers or those going from one end of the earth to the other without resighting where deflection reverses so compound effect accumulates.

 

I include some basic facts below but its really stuff for academic interest to most of us.

 

You can get it calculated by Bulletflight Level 3.

 

Coriolis Effect - Basic Facts

 

Predictable & Calculable but typically obscured by wind call errors.

Independent Horizontal & Vertical components.

Minor so ignore except at extreme range & if you have time to factor it in.

 

Horizontal deflection is …

 

Determined by Latitude.

Unaffected by direction of fire.

RIGHT in Northern Hemisphere.

LEFT in Southern Hemisphere.

Minimal at Equator / Maximum at Poles.

 

The VERTICAL component depends upon Latitude & Direction of Fire.

 

East shot will go High.

West shot will go LOW.

North or South shot has NO vertical component regardless of latitude.

Max. effect at directions parallel to & near Equator.

Is an acceleration like gravity which means we can calculate it as an adjusted drop.

[brown dog]

Attached what may be a useful read.

 

Because it's talking about gunnery (where this stuff matters) you'll need to do a smidge of mental translation - eg shot going LONG or OVERRANGE is the same as a rifle shot going HIGH etc.

Coriolis extractt2.pdf

[rabbit fingers]

This may help to visualize the effect.

interesting read bd what's on the other 54pages?

[brown dog]

This may help to visualize the effect.

interesting read bd what's on the other 54pages?

 

Graet vid! (and I could tell you, but then you'd have to jump into a shredder )

[rabbit fingers]

Spoil sport. not like I'd tell anyone.

[gun nut]

Having watched the video and read the text I've decided to sneak either due north or south of anything I want to hit that's a long way

away, cos it'd be easier than the maths!

[6.5shooter]

Thanks for the info lads!

In my original post I was referring to one shot hits, Im more of a vermin shooter and long range plate basher instead of shooting comps so I try and make alot of effort into trying to calculate/guestimate where my first shot will land.

Just wondered if anyone actually bothers to allow for this.

 

Garry

[DaveT]

Thanks for the info lads!

In my original post I was referring to one shot hits, Im more of a vermin shooter and long range plate basher instead of shooting comps so I try and make alot of effort into trying to calculate/guestimate where my first shot will land.

Just wondered if anyone actually bothers to allow for this.

 

Garry

 

My view is as before...... only at extreme range (over 1000 yards) and if you have plenty of time and you know that your wind-call is not going to make Coriolis allowance irrelevant.

 

Its still worth knowing what it is & how to calculate it from pure interest BUT ... why not let a ballistics PGM do it for you if you really want it?

 

Dave

[rabbit fingers]

Im more of a vermin shooter and long range plate basher instead of shooting comps

 

Garry

Me too. interesting subject but in reality it gets lost in all the chatter of everything else that affects the bullet over that distance. coriolis effect is equivalent to a VERY small change in the wind. What turret adjustments do you have? They need to be pretty fine to dial it out anyway.

[gbal]

Having watched the video and read the text I've decided to sneak either due north or south of anything I want to hit that's a long way

away, cos it'd be easier than the maths!

Why not just bypass any Coreolis Effect,and stalk to an east-west,or west east position for the shot,then there is no CE effect at all.It only applies if the target is displaced N or S,from your position.But remember the Spin Drift from your barrel's rifling will still apply,though it' s quite small say 1/4 moa (depends on the twist/spin rate).Can't be having things too easy,can we!!

george

[gbal]

Me too. interesting subject but in reality it gets lost in all the chatter of everything else that affects the bullet over that distance. coriolis effect is equivalent to a VERY small change in the wind. What turret adjustments do you have? They need to be pretty fine to dial it out anyway.

Right,as dave T and others said.You can forget about Spin Drift too-its in the 1/4 moa zone.A mistake of 1mph for a 10 mph wind will be a bigger factor than most of the other suspects-and who can actually reliable manage 1mph accuracy?In the worst case scenario,all the errors are in the same direction,so are cumulative.I think, paradoxically,if you rifle doesn't group that well,you might still get lucky with a flier!But better to go with consistency,and have 5 misses in the same hole,probably.Or get a shotgun.

george

[baldie]

The fart from last nights curry will have more effect on your shot.

 

Worry about things that matter, like getting a very accurate wind call each shot.

[gbal]

Thanks for the info lads!

In my original post I was referring to one shot hits, Im more of a vermin shooter and long range plate basher instead of shooting comps so I try and make alot of effort into trying to calculate/guestimate where my first shot will land.

Just wondered if anyone actually bothers to allow for this.

 

Garry

Quite right to take care Garry-the target guys of course have the luxury of a sighter-which includes all the drift that applies from any factor!Long range US varminteres can typically see bullet splash,to the same advantage.As Dave T advises,a good program will handle all this,and spin drift,and temperature etc etc,but ALAS nothing will really measure wind accurately enough to reduce this,the main error source.Spin drift eg might work with wind/coreolis to accumulate error,or partly mitigate some of the error-depending where you are in the world,and which compass point you are shooting to,and which way the wind is blowing,and your twist.

All very complex,but thankfully,wind apart,effects are generally very small,and bearing in mind the not completely predictable spread of shot from even an accurate rifle (1/4 moa,probably proportionally not quite that good as range increases)might in effect cover it!Sometimes.

george

[rabbit fingers]

Spin drift has a bit more effect. shame they are stacked. maybe we in the northern hemisphere should build rifles with a left twist to counter coriolis. we'll have to get the actions with a left hand thread too or well have the barrels unscrewing. can't see it catching on somehow

[gun nut]

What is the max Coriolis effect on a 400yd shot? If its .25moa up down left or right it's not going to cause a miss at anything I'm likely to be trying to harm. Interesting topic but I'm just getting my head around wind dopping without another variable to confuse the issue. A complete novice i had out last year shot a 300yd rabbit with the 20tac. When I asked him about his drop and wind allowance he said "what's an allowance, I just put the cross in the middle and squeezed like you told me"

 

Mike

[brown dog]

Just to be a full nerd; nerdies don't call spindrift 'spindrift', it's called 'drift'.

"Drift" is what's caused by spin.

To distinguish it from what the wind does causing the bullet to move bullet sideways, that's called 'deflection'.

 

 

Quantities?

 

Coriolis; utterly pointless in a direct fire weapon system. Changes according to the bearing of fire and everytime you move N or S on the planet; and it's tiny at direct fire distances.

 

(Spin)drift; bigger than's been stated. Reckon on a 308 being about 1 MOA at 1000yds. 10". It's always a left correction. And it's always there and it's easy to work out.

If I told you your 1000yd zero was off by 10" right; would you reset your sight or just say 'sod it'?

I can't understand anyone saying 'ignore (spin)drift' for shots beyond about 5-600m. [i suspect that's mainly people who get sighters for whatever they're doing]

[gun nut]

Now that's worth lookig at! It can't be 1moa all the way out is it? ie 4" at 400yds? And is it more for higher velocity rounds with faster twists ie .270win which is typically 3100fps out of a 1/10 twist? And doesn't it depend on L-R or R-L wind with one giving a drift to 2o'clock and the other a drift to 7o'clock? Or have i got the wrong spin on this?

 

Mike

[brown dog]

Now that's worth lookig at! It can't be 1moa all the way out is it? ie 4" at 400yds? And is it more for higher velocity rounds with faster twists ie .270win which is typically 3100fps out of a 1/10 twist? And doesn't it depend on L-R or R-L wind with one giving a drift to 2o'clock and the other a drift to 7o'clock? Or have i got the wrong spin on this?

 

Mike

 

Mike here's a fag packet approximator (it's stunningly close to working it out properly if you get the constant right):

 

Drift[inches] = (constant * TOF^2)/2.54

 

 

EG Using a constant of 11 for a 155gr 308 in a 1/12 barrel

 

Range----TOF (sec)----Drift (inches)

100m-------0.1------------0.04

400m-------0.6------------1.56

800m-------1.4------------8.49

920m-------1.7------------12.52

 

The constant generally lies between 10 and 20.

 

 

Is it more for faster bullets? Maybe, maybe not I'm afraid; it's actually a stabilisation question - a bullet of any cartridge that's over stabilised will probably drift more than one of any cartridge that's properly stabilised to have it's nose follow the trajectory.

 

 

 

As regards the 2 oclock 7 oclock thing, you're starting to bring in another (different) effect that causes lift (magnus).

[gun nut]

Thanks BD, where would you find this constant or is there a simple formula for it? Presumably the drift is to the right with a RH twist barrel and to the left with a LH twist barrel? And the faster the bullet spins the higher the constant, so long for caliber bullets at high rpm would be about 14-17 presumably or am i over simplifying it? I'm thinking particularly of vld bergers in 6.5mm here.

Starting to wonder how the hell I ever hit anything at all! This does however explain the 4- 6" of "wind" at 600m that I always seem to be out by on the sighter, even on a calm day!!

And the magnus effect is a vertical component only, so a combined drift and magnus effect would be 11o'clock for a R-L wind if the wind deflection is slightly more than the drift, if you get my drift?

 

Mike

[brown dog]

Thanks BD, where would you find this constant or is there a simple formula for it? Presumably the drift is to the right with a RH twist barrel and to the left with a LH twist barrel? And the faster the bullet spins the higher the constant, so long for caliber bullets at high rpm would be about 14-17 presumably or am i over simplifying it? I'm thinking particularly of vld bergers in 6.5mm here.

Starting to wonder how the hell I ever hit anything at all! This does however explain the 4- 6" of "wind" at 600m that I always seem to be out by on the sighter, even on a calm day!!

And the magnus effect is a vertical component only, so a combined drift and magnus effect would be 11o'clock for a R-L wind if the wind deflection is slightly more than the drift, if you get my drift?

 

Mike

 

Mike, there's no way of working out the constant for this method other than matching the output to empirical data.

 

So...your observation that you're always 4-6" (so, 5") out at 600m on a no wind day is perfect:

 

Look up your TOF at 600m and put it in the formula; then play with the constant until it gives you an answer of 5"; the number you find will be the constant for that bullet from your rifle and you can use it to calculate drift for all other ranges and TOFs for that bullet from that rifle.

 

It's a crazy-accurate approximation when done well; compared to doing the 'full non-empirical maths' based on calculated stability factors, the approximation will be within fractions of an inch of the same answer.

 

Yes drift to the right for clockwise twist barrels (I think only the Russians make them with anti-clockwise twist!).

 

There's no rule of thumb relating to vel; in any calibre if I was to guess at one, a bullet that's being spun faster that 'conventional' for it's length will need a higher number; a bullet that's not been spun fast enough will be lower.

 

I wouldn't think about trying to visualise all the effects in a 'oner'. Drift moves your MPI to the right. Then consider the distribution of impacts about that MPI due to deflection and magnus in the normal 'diagonal' way.

[gbal]

Mike, there's no way of working out the constant for this method other than matching the output to empirical data.

 

So...your observation that you're always 4-6" (so, 5") out at 600m on a no wind day is perfect:

 

Look up your TOF at 600m and put it in the formula; then play with the constant until it gives you an answer of 5"; the number you find will be the constant for that bullet from your rifle and you can use it to calculate drift for all other ranges and TOFs for that bullet from that rifle.

 

It's a crazy-accurate approximation when done well; compared to doing the 'full non-empirical maths' based on calculated stability factors, the approximation will be within fractions of an inch of the same answer.

 

Yes drift to the right for clockwise twist barrels (I think only the Russians make them with anti-clockwise twist!).

 

There's no rule of thumb relating to vel; in any calibre if I was to guess at one, a bullet that's being spun faster that 'conventional' for it's length will need a higher number; a bullet that's not been spun fast enough will be lower.

 

I wouldn't think about trying to visualise all the effects in a 'oner'. Drift moves your MPI to the right. Then consider the distribution of impacts about that MPI due to deflection and magnus in the normal 'diagonal' way.

 

OK,it's complex.Would an easy partial solution,reducing the effect especially at shorter ranges,be to sight in at say 250/300 yards(and at least know what your scope settings then are,so you can set to that anytime),then l/r 'error' will be much reduced -though there -out to say 600? Once zeroed of course it should not change at the zero range.The convenient practice of zeroing at 100, tends to mean the smaller effects are not really picked up,as they haven't had time to be significant.Longer zero too means less vertical,though that isn't so crucial as its known and dialable.Alternatively,for Spin Drift,its a few clicks ( whatever you are persuaded SDrift effect is in your rifle) more compensation than for the wind alone,if wind is l to r,and less if r to l.It's not hard to reduce error,and it is as ever,no bad idea to feel you have made every reasonable effort.You will still make some wind error judgement usually,but reducing the knowable effects is helpful.

[gun nut]

Cheers BD, on the range tomorrow so will give it a whirl, mind you we're forecast for 15-20mph westerly's tomorrow on a north-south range so deflections will be substantial anyway.

 

Mike

[DaveT]

Cheers BD, on the range tomorrow so will give it a whirl, mind you we're forecast for 15-20mph westerly's tomorrow on a north-south range so deflections will be substantial anyway.

 

Mike

 

 

In that case (Har har) and just to add a little more 'spice' ... don't forget that a FV Westerly is going to DEPRESS your shot by a suprising amount as well as deflect to the right. Have fun!

 

Dave T

[rabbit fingers]

In that case (Har har) and just to add a little more 'spice' ... don't forget that a FV Westerly is going to DEPRESS your shot by a suprising amount as well as deflect to the right. Have fun!

 

Dave T

 

Wouldn't a shot north -south in a wind from the west lift the poi ?