Coriolis Effect - practical considerations

[bartman007]

Here is a question to all those super technical shooters out there, and laypersons like me:

It has been pointed out to me on numerous occasions that one should take into account the effect of Coriolis when shooting long range.

Given the fact that Coriolis is the effect of an object moving relative to the rotation of the Earth, how does one use this scientific fact when competing around Australia?

The reason I ask this, is that each range faces different directions, and are at different distances from the equator. So to make a judgement that it is worth 1 point left at 1000 yards can be the total opposite when shooting from the butts back to the mound on a North South facing range. On an East West range, we will be affected mainly by elevation instead.

Is it just simpler to put ones head in the sand, and just shoot from your 300 yard zero?

[Brad Y]

You have been watching too many you tube videos on how snipers make 2 mile kills havent you mike

[AlanF]

Mike,

In terms of team shooting, a key priority is to have identical zeros across all team rifles. If its done side by side at a shorter range, and rifle ballistics are similar, then any general effects at long range such as Coriolis will apply equally to all rifles, so will not cause problems.

Alan

[bartman007]

If its done side by side at a shorter range, and rifle ballistics are similar, then any general effects at long range such as Coriolis will apply equally to all rifles, so will not cause problems.

Yes I tend to agree Alan. And in regard to Team Shooting, we get to practice on the day before, so setting a zero becomes easy.

The key indicator for me is that my zero changes slightly at different Rifle Ranges. This is normally accounted for at the first range during sighters.

However, due to the effect being based on the direction of the range, then the majority of shooters probably ignore Coriolis altogether.

Or are there some very clever shooters out there that use google earth and the like to determine the angle of the dangle and make the corrections prior to shooting at a new range? Like when you travel for OPM's.

[DaveMc]

Mike - Horizontal Coriolis has nothing to do with direction you are facing. It is to do with latitude.

Think of it this way - if coriolis was opposite when facing north to facing south then we would never get a cyclonic clockwise spin.... - it has to bend to the left in all directions (in Southern hemisphere).

Going from Australia to Raton equated to a significant effect on the 1000 yard wind zero (4 inches at 1000 yards) as it changed from left drift to right. But generally speaking anywhere in Australia will only equate to about an inch difference at 1000 yards. Worst case going from Cairns or Darwin to Tasmania (an extra click at 1000 probably).

The formula for calculating is (0.00007292 * (distance in feet)squared * sin (lat)) divided by average velocity (in fps).

1000yard calc :so according to Geoffs et my time of flight is around 1.25 seconds for 1000 yards (average of 2400 fps) In cairns coriolis would be (0.00007292*3000*3000*sin(16))/2400 = .075 feet = 0.9 inches to the left

in Tassie Campbeltown is almost 42 S so = 0.00007292*3000*3000*sin (42)/2400 (if same temp and air density). = 0.1826 feet = 2.19 inches

[DenisA]

Is it actually worth figuring out given that we have 2 sighters and can utilise them in a mild condition and a heavier condition or whatever strategies called for on the day.

I can understand snipers needing to know for the all important one shot, but that's a completely different game in all aspects.

I agree its important to understand the effect is there, but like spindrift, its accounted for in sighters.

[DaveMc]

So Denis - what do you do when you think there is 1.5 minutes on from the left but your wind zero is 1/2 a minute out. No problem - 2 sighters put you back in the middle but you now think this is actually 2 minutes of wind. What if the wind switches through centre with the same strength on the other side (actually only 1.5 minutes but you now think it is 2). You crank on 2 minutes only to find yourself a full minute out (double the error - You drop at least 1 maybe 2 points.

[bartman007]

Dave, I have never seen the formula, however from a Youtube video I saw (and Youtube is always right ), a person shot a group over 1000 yards firing East then West.

His groups varied in elevation as I'd expect when taking into account the spin of the earth relative to the shot taking off and landing 1000 yards away. Firing against the spin of the Earth would logically raise the shot compared to firing with the spin of the Earth. Firing at right angles to the spin of the Earth would either throw the shot left or right depending on your orientation.

Your formula is intersting, and as the results say, it really only accounts for about 1.3" which is insignificant

DenisA, while sighters are a handy tool to getting into the Center, I shot at Canberra in 2010 when the conditions were constant fishtail. I knew my rifle was zero'd when I left home (Victoria), but kept missing the Center as the wind went left and right. At the time I couldn't give a good value of wind when I was sighting (I didn't read the strength well back then), so was never able to work out my Canberra wind zero. Subsequently I lost shots every time the wind changed directions. Could it have been Coriolis?

[bartman007]

So now you add spin drift.

DOH, I'm glad you mentioned it now, I'll factor that in when down under under.

+1 on Dave's comments about sighters giving you an incorrect view of wind strength. Skewing your view of true wind zero!

[DenisA]

So Denis - what do you do when you think there is 1.5 minutes on from the left but your wind zero is 1/2 a minute out. No problem - 2 sighters put you back in the middle but you now think this is actually 2 minutes of wind. What if the wind switches through centre with the same strength on the other side (actually only 1.5 minutes but you now think it is 2). You crank on 2 minutes only to find yourself a full minute out (double the error - You drop at least 1 maybe 2 points.

Coriolis plus spin drift will be in the order of 3/4 to 1 minute at 1000 yards in Tassie right of your 300 yard zero. If you are happy shooting with that then go ahead - for me it is critical to understand this and an important part of individual and team shooting.

Yep, I've been caught like that a few times. That feeling of zero confidence at that point is unsettling and the dropped point are devastating. Should have thought a little harder before asking. Thanks for the example. Don't know why such a simple thing has eluded me. Definitely see that its critical to understand now.


Edit: Glad I asked though, 1 step closer to never dropping another point

[DaveMc]

Dave, I have never seen the formula, however from a Youtube video I saw (and Youtube is always right ), a person shot a group over 1000 yards firing East then West.

His groups varied in elevation as I'd expect when taking into account the spin of the earth relative to the shot taking off and landing 1000 yards away. Firing against the spin of the Earth would logically raise the shot compared to firing with the spin of the Earth. Firing at right angles to the spin of the Earth would either throw the shot left or right depending on your orientation.

Your formula is intersting, and as the results say, it really only accounts for about 1.3" which is insignificant

?

Mike - Vertical coriolis effect does effect height depending on direction (sorry about my previous comment) but not horizontal direction

[bsouthernau]

Mike

It's far from intuitive but whatever the direction of travel Coriolis "acceleration" is always perpendicular to the direction of travel and to the left in Southern Hemisphere and right in the Northern. At any instant its magnitude is
2 x (earth's rotational speed) x (sine of the latitude) x (bullet speed)

It all arises because our frame of reference (earth's surface) is not fixed but is itself accelerating. So the bullet heads off with its initial velocity and the earth rotates away underneath it giving rise to this apparent deflection.

Barry

[DaveMc]

Sorry guys, Barry is correct in direction so I have edited my posts above to be correct.

[AlanF]

This could be further complicated by reports of a vertical component of Coriolis effect on a projectile which IS dependent on direction of the shot. I would strongly recommend that all teams (other than Vic who have been researching this for months) give this top priority at all times while you are in Campbell Town.

Alan

[bsouthernau]

This could be further complicated by reports of a vertical component of Coriolis effect on a projectile which IS dependent on direction of the shot. I would strongly recommend that all teams (other than Vic who have been researching this for months) give this top priority at all times while you are in Campbell Town.

Alan

Too late by then Alan - their cause is lost. Don't know why they're even bothering to go to Tassie.