Velocity Loss In DIMPLED Sub-Sonic Projectiles:

This Was An April Fools Day Joke Image - but ..

PLEASE don't shout at me here if you know the answers to my conjectures .. because I'm a modern person who may become offended, tragically disadvantaged and litigious 😃.

Recently thinking about slingshot shooting pointed my head at spherical sub-sonic projectiles -  I read that balls fired from catapults / slingshots may achieve up to 190-200 feet per second.

But it's a sad fact that a spherical ball loses speed about three times faster than modern "pointed" bullets do on average. Therefore, the effective range of those earlier weapons is much less than a modern firearm.

 - whereas my very quiet pistol calibre projectiles from a well silenced firearm might be flying at perhaps 800 ft./sec.

Scientifically - anything below 1125 feet per second is SUB-SONIC at sea level .. (767mph)

- Now step sideways from the range onto a golf course and it seems that the USGA and the R&A (.. the European golf governing body) have a rule that the velocity of the golf ball shall not be greater than 250 ft. per second. 

Golfers have long opted for DIMPLED balls because they hold their velocity better - so can travel further. - When testing golfing equipment - the USGA uses a swing speed of 109 mph to simulate a pro's swing speed & this results in an actual golf ball top "muzzle velocity" of around 180 mph  (264 ft/sec). 

FORE 

Now the smarter readers will have noted that both golf balls and slingshot ball ammunition are mostly departing at similar velocities of around 190/200 foot per second AND both may cause serious injury upon impact.

If you want to get deeper into the aerodynamics, there are two types of flow around an object: laminar and turbulent. Laminar flow has less drag, but it is also prone to a phenomenon called "separation." Once separation of a laminar boundary layer occurs, drag rises dramatically because of eddies that form in the gap. Turbulent flow has more drag initially but also better adhesion, and therefore is less prone to separation. Therefore, if the shape of an object is such that separation occurs easily, it is better to turbulate the boundary layer (at the slight cost of increased drag) in order to increase adhesion and reduce eddies (which means a significant reduction in drag). Dimples on golf balls turbulate that boundary layer.

The dimples on a golf ball are simply a way of creating turbulence in that boundary layer.

Yes I fully understand that normal bullets are supersonic therefor none of this applies to them .. but might there be an argument to dimple the surface of elastic driven slingshot steel balls and maybe other sub-sonic projectiles such as those fired from captive piston cartridges such as the Russian silent SP-4.

My further guess is that a dimpled surface should reduce metal to metal friction losses as this type of projectile progresses up the barrel.

- Going back to our round balls or spherical shot - there is an accuracy issue that results from spin of a spherical projectile - known as the MAGNUS EFFECT .. where a ball can be seen to curve away from it's path as the result of lift generated by spin - such as when a golf ball is mishit or "sliced" ..

- Spin shouldn't cause too much problems for slingshooters but,

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Just to add some perspective .. Russia has announced deployment of their new nukes missile system AVONGARD that can fly at 20 - 27 times the speed of sound - delivering 2x megaton nuclear war from 27 December 2019 - at something over 12,000 - 19,000 mph .. meanwhile NZ police are prohibiting & seizing our .22" rabbit rifles with 11+ round magazines.

Cheers

Marty K.