Author Topic: Top small bore shooters seem to prefer low velocity (e.g. 270 m/s) .22LR  (Read 7335 times)

admin

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... the reason I heard from an offical trainer is that the wind does affect less.

this is in contrast to the theories I know?

mman

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No this is exactly in line with the theory.  The main reason to this effect is that wind drift depends on drag coefficient. And this coefficient is lower at subsonic velocities for most bullets. See 22 lr. wind study by McCoy regarding this matter.

For past few days I have tried to model sub and supersonic air flow of bullets with CFD. If I can calibrate the model to give accurate results there is no more use to do time consuming and expensive drag measurents.
« Last Edit: March 02, 2012, 12:15:11 PM by mman »

admin

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No this is exactly in line with the theory.  The main reason to this effect is that wind drift depends on drag coefficient. And this coefficient is lower at subsonic velocities for most bullets. See 22 lr. wind study by McCoy regarding this matter.

True but the faster bullet is less in the wind... If one does a monte carlo, (e.g. what do i shoot today spreadsheet) the higher accuracy prefers a higher velocity. I am going to recheck

admin

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For past few days I have tried to model sub and supersonic air flow of bullets with CFD. If I can calibrate the model to give accurate results there is no more use to do time consuming and expensive drag measurents.

Cool!

mman

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Analytical approx. equation for wind drift is:

(A*C*L^2*p*w)/(4*m*v) where

A = bullet sectional area
C = drag coefficient
L = shooting range
p = air density
w = wind velocity
m = bullet mass
v = average bullet velocity

If A, L, p, w and m are constants then wind drift depends just on C/v. So if slower bullet drifts less then C/v must be less for lower velocities. And if you look the graph below that is exactly the case for RA4 projectile which is pretty good fit for most 22 lr. bullets. Optimum muzzle velocity for wind drift is usually about 250 - 300 m/s for any subsonic projectile.



Here in Finland few benchrest guys (benchrest.fi) have done extensive empirical testing for rimfire wind drift charasterics. They claim that there is clear difference in wind drift with different barrels and different ammo lots although muzzle velocity would be the same. This is interesting but I must say I'm a bit skeptical for this results. Not to say that it could not also be true. Rifling grooves and initial yaw have an effect on drag but is it enough to see at the target I don't know. At least McCoy did not get different drag coefficients for different barrels.
« Last Edit: March 02, 2012, 09:09:26 PM by mman »

admin

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Yes I understand, but the simulations show that velocity wins !

mman

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Wins if wind is only dispersion factor? Then there must be an error on the code?

admin

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Well I guess you have to look at the spreadsheet to see what it takes into account: wind, wind flucutions, shooter accuracy, muzzle velocity fluctuations...