Coriolis effect...again...

Direction of spin drift??

When using the JBM calculator for spin drift, which way is the bullet path going to drift? If it is right hand twist will it drift towards the right? That is intuitive, but is it correct? I never realized that spin drift existed until this thread came along. Sheesh---just when it seemed like things were getting easier..... :)
 
LOVE this thread! It has added so much to my vocabulary.

Now I have many more plausible reasons that fit:

"I missed the target because________.:)"
 
Just a bit of trivia: The Finnish Mosin-Nagant, and likely some other rifles of that era, had spin-drift correction built into the iron sights.

It's actually pretty cool.

Josh
 
"The direction of the deflection is toward the direction of the spin...."

Which is why, in the Northern Hemisphere, you should use a left-twist barrel, and cant the rifle a couple degrees to the left, to minimize the coriolis effect effect.
 
Which is why, in the Northern Hemisphere, you should use a left-twist barrel, and cant the rifle a couple degrees to the left, to minimize the coriolis effect effect.
Click to expand...

The problem with that is that the Coriolis effect is different when you are shooting east or west than when you are shooting north or south.

Coriolis effect is nothing more than the fact that since the earth is turning, we are in fact shooting at moving targets from a moving shooting station.
 
I wouldn't say negligible, but appears to be somewhere around half as much influence. Of course, depends on direction of fire.

Another Gunwerks article below provides a little more info, along with a suggestion to zero at long-range in a N/S direction. This would largely account for spin drift, leaving Coriolis drop changes which the shooter could account for- again, significant most when shooting due E or W.

The long-range zero is an interesting suggestion, but I think only suitable on a zero wind day to be viable. My winds calls still leave a lot to be desired and would no doubt result in an incorrect zero.

I checked a sat image of the range we frequent, and looks like it's about 330 degrees, so not too bad in terms of correction needed for drop.

Magnus spin drift will cause a bullet to drift right for a right hand twist barrel, and the amount is proportional to the square of the time of flight. Again, BC and velocity is important. We've measured the reference system to be on the order of eight inches at 1000 yards, but because of its exponential dependency on time of flight the 1500 yard drift could be as much as five feet!

Corriolis effects from the earths rotation also needs to be compensated for. We will ignore any left/right effect, and focus on the vertical errors. At 1000 yards the rotation of the earth is basically moving your target down 2.6 inches per second if you are pointing east, and up if you are pointing west. Again we see how important a short time of flight is to minimize these effects. Our reference system will take 1.3 seconds to reach 1000 yards. That adds up to a difference of point of impact between shooting east and west of nearly seven inches. We can minimize the effects from Magnus and Corriolis for sub 1000 yard shooting by zeroing at a longer range like 800 yards—while pointing north or south. Then our errors are within the accuracy potential of our hunting rounds.
Click to expand...
 
Run the JBM and/or QuickLoad calculations -- which take everything into account starting with bullet, BC, weight, velocity, location, direction, etc
Worst coriolis case is (literally) a couple of inches at 1,000, which is even then totally dependent on lattitude & direction of fire.

Spin drift, on the other hand, can be measured in feet & fractions thereof -- and is always with you no matter what direction or location.
 
I would think that worst case Coriolis effect would be a shooter located at either the north or south pole of the earth. Since the earth spins one revolution per 24 hours, or 15 degrees per hour, or 15 moa per minute, the target would move 1/4 moa for each second it took for the bullet to reach the target. Figure a 1.6 second time of flight for a 1000 yard shot and that's about .4 moa of movement or about 4 inches at 1000 yards. Not really much compared to wind drift and other stuff.
Now if you are shooting at something many miles away with artillery where the time of flight is minutes, not seconds, you do have to account for Corialis effect.
For example, the time of flight for a 16 inch naval gun shooting at a target 24 miles away is said to be about 90 seconds or a minute and a half.
That would give a target time enough to move 22.5 moa and 22.5 moa at 24 miles is 829 ft.
 
You must log in or register to reply here.
Back
Top