spin rate

  • Last Post 02 December 2019
  • Topic Is Solved
Ross Smith posted this 01 December 2019

I obviously still don't understand why long bullets need to be spun(twist rate) faster than short bullets. For instance arrows don't spin very fast and the 30BR loaded with 120gr bullets in a  1:18 twist barrel.

Any help?

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RicinYakima posted this 01 December 2019

One reason is due to angular momentum.

When a body is spinning around some axis, we say the angular momentum is pointing along this axis with a magnitude equal to the magnitude of the angular momentum. When a body has angular momentum is some direction, much like with regular momentum, it doesn't like to have its magnitude or direction changed.

One particular note to this is precession, where the angular momentum of a body spins about a 3rd axis, but it doesn't like to have this 3rd axis changed either.

So, when the bullet has some spin, it has some angular momentum in the direction of its motion. This spinning adds stability, because the bullet itself doesn't want to turn on some other axis, thus changing the direction of its angular momentum, so it stays pointing straight. Since it stays pointing straight, it is more aerodynamic, so it flies for longer.

The longer the bullet, the more momentum must be created.. This momentum is created by a  faster twist; calculated by length compared to diameter of bullet. A 500 grain bullet will shoot just fine in a 45/70 Trapdoor (1 in 20) because it is short compared to its diameter. A 220 grain bullet has to have a 1 in 10 to shoot from a Krag.

Arrows are long and thin; however the fletching maintains constant pressure to keep the arrow turning as long as it has air flow.

HTH;s Ric

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Ross Smith posted this 01 December 2019

Thanks Ric, that helps but I'll hav to cogitate a while.

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RicinYakima posted this 02 December 2019

If it was easy, I would have majored in Physics instead of Chemistry! I hated all that math.

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Ross Smith posted this 02 December 2019

I always liked the physical science part of physics but the advanced part overwhelmed me, so I'm an old biology major. Any way, the one situation that makes me wonder is the 30BR with a 1:17 or 1:18 twist. They really whistle that bullet along too.I have 2 30 cal barrels with 14 twist and they do not like the same bullet or velocity range. Throating aside that is. They definitely have different throats.

Is there a video some where that shows that 3rd axis?

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