Bent - pellet marking for spin rate tests

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Yrrah

Yrrah

Member
Apr 1, 2015
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NSW, Australia
Bent, Prussian blue would be OK.
I used ink taken from a ball point pen to mark the heads with a carefully applied thin stripe from near the centre of the head to the rim of the head. 
Index the stripe to say 12:00 o'clock when loading. 
Be very careful / precise when geometrically orienting the recording cards relative to each other; and carefully measure the gaps between them. 
Make sure to determine the "rifling" direction as LH or RH because you must know which way the pellet is spinning when you come to do the maths/geometry.
As with most things, the accuracy of the set-up determines the quality of results.
I used a protractor, Dan used Photoshop and not sure what Fredrik used to measure the angles - we are the only ones I know of who have done this with the ST barrels.
With ST barrels you will find that different weight pellets will give you different results because the amount of spin rate will be affected by the momentum of the pellet as it strips through the "rifling" in that last 1-1/2" at the muzzle.
Here is the result from another study I did with conventional rifled BSA .25 Hornet to measure spin rates/unit travel out to 200 yards .

The report from that experiment http://www.network54.com/Forum/79537/thread/1281248469

... Enjoy the process. Regards, Harry.

 
"Bluestone"Excellent and innovative study.

A question if I may. Would reducing traditional, full barrel, rifling twist rate result in better long range accuracy but at the expense of accuracy at shorter ranges ?
Click to expand...
Bluestone,
I very much doubt that it would. 
In the powder gun/ bullet shooting world it may. Reducing the rate meaning of course makes it slower as in going from say 1:12" to 1:14". I understand that the powder shooters who specialize in long range target work are now tending towards a slightly slower twist rate (TR) for better accuracy at the 600- 1000 yards and that it is at the expense of fractionally less accuracy at the closer ranges.t
However, bullets do not share the advantage pellets have of being predominantly drag stabilized; a characteristic which is most in evidence at relatively short ranges when shooting our air rifles.
So, a slower pellet spin rate (SR) induced by a slower TR (Edit: combined with the predominant drag stabilization) should be an advantage and the best of both worlds for us. (LATE EDIT for clarity: Drag stability when the velocity is high at close range for good accuracy, and throughout whole flight; and just enough spin to ensure stability when fighting any destabilizing forces; but not so much spin that dynamic stability is lost resulting in spiral flight out at ranges where gravity is fighting with static stability, thus preserving tractability).
That a slow pellet spin rate can deliver the goods with a well designed pellet can be assessed in these two SM videos I took a few years ago now when I had established the actual spin rate of .25 Kings from the ST barrelled FX Elite was around 1:75 inches not 1:16.
Accuracy and stability (lack of precession or spiral) over 50 yards in a light L to R wind:
http://vidmg.photobucket.com/albums/v392/Kyogle/FX25STElite3JSBKings51ydspigsilhouetteJan2012wmvforEmail_zpsacc2791c.mp4

Similarly a lack of precession or spiral over 71 yards of travel, and accuracy on impact:

http://vidmg.photobucket.com/albums/v392/Kyogle/FXElite5Kings71yardsJan2012-1.mp4

So much for relatively short ranges.
Now for long range at 183 yards we see pics of Kings showing how truly point first they arrived at that range from an initial spin rate of 1:75".
Pellet shot into dirt at 183 yards to show point-on impact:

Shot onto hardwood tree:

Shot through one layer of dead bark into the fresh bark indicating penetration stability:

Then the one and only group that was shot at the same 183 yards in that session:


The peerless inventor of the Whiscombe powerful springers, John Whiscombe, experimented with slower twist conventional barrels and I understand settled on a one in 30 something inches for some of his highly sought after hand made rifles.

I hope what I have gathered for you lends support to the idea that slowing the spin rate does not necessarily come with reduced accuracy or reduced pellet stability neither at relatively close or long range. ....... Kind regards, Harry.



 
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