Velocity and bullet spin

[hogwire]

I feel that velocity in a given twist barrel effects the rate of bullet spin. A bullet maker told me that it doesn't. The rate of spin is only effected by rate of twist. Does anyone have the math or proof of this? Make it simple please.

[Robinhood]

If the rate is one turn in 10 inches then that is what it is spinning as it leaves the barrel. If you speed it up or slow it down it still is spinning at that rate when it leaves the barrel within a given distance. If you increase or decrease the velocity then the RPM or RPS will increase but so will the distance traveled in a given time frame. Two different animals however, velocity in a given twist with regards to bullet length does affect stability.

Here is a tool from Berger that will help with your understanding if you take time to play with it.
http://www.bergerbullets.com/twist-rate-calculator/

[Stumpkiller]

Rate of spin stays the same. But revolutions per minute increase with velocity.

A friend was shooting light bullets in his .22-250 and having horrible results. Sometimes missing the target sheet at 100 yards. He moved the target up to 25 yards and blew it into pieces. Turns out his 50 grain SX bullets were vaporizing into pieces. We figured they were leaving the muzzle spinning 270,000 rpm, or in that neighborhood! That was ripping the jackets off the cores! That's like eight times faster than a Dremel tool

[hogwire]

Thank you very much! Your answer was easy for me to understand and that Berger tool was the icing on the cake. Velocity effects the stability but more because of the BC more than the twist. There are just so many things going on. Too many rpms cause bullets to fly apart but you can slow them down and stop the destruction. Hmmm, it seems the smarter I get the more stuff I don't know! Thanks to Robinhood and Stumpkiller for their comments!

[Robinhood]

Hmmm, it seems the smarter I get the more stuff I don't know!

Yep.

[Texas10]

I don't quite see it that way, but maybe I'm wrong. When the bullet leaves the barrel it begins decelerating but the rotation rate decays much slower. So towards the end of flight, the bullet is traveling, let's say 1000 fps and the rotation could be still going 200,000 rpm (3,333 rps) or 3.33 revs per foot of travel.

[Robinhood]

Wouldn't that position destroy transonic instability? I'm thinking universal principle of decay. Entropy if you will. To maintain spin at a constant you would have to continue to apply energy. Thoughts......

[lonestardiver]

You have angular momentum for the spin. Think of a top spinning and how it spins for quite a while. It is in addition to the forward momentum that gets acted upon by drag (air resistance).

[Texas10]

Wouldn't that position destroy transonic instability? I'm thinking universal principle of decay. Entropy if you will. To maintain spin at a constant you would have to continue to apply energy. Thoughts......

Bullet spin is not at a constant rate, it does decay but just much slower compared to the decay of bullet velocity due to atmospheric drag.

Have you seen the videos of bullets being fired from a handgun into a frozen lake? Bullet spins like a top on top the ice. An exaggerated example but the properties are the same.

Maybe we should get the Slo Mo guys to video a bullet a 1000 yds and count the revolutions per foot of travel!

[JW]

Now my head is spinning lol
just shoot em
Jack

[Robinhood]

Maybe we should get the Slo Mo guys to video a bullet a 1000 yds and count the revolutions per foot of travel!

Ready....Go.

Great analogy on the bullet spinning on ice.

Does B.C. effect the rpm's at range?

[Texas10]

That's an interesting question. I see your question and raise you another. As the rate of spin increases relative to the bullet velocity, is there a sweet spot where stability is maximized, say at 1.333 or more revs per foot of travel and that this is when the bullet "goes to sleep", stops wobbling, and reaches max B.C?

Slo Mo guys, are you there?

[Stumpkiller]

There is definitely a "sweet spot" for any given bullet from a cartridge. But manufacturers settle on a medium rate to handle a variety and range of loads.

.223 Rem/5.56mm used to be 1:12", then 1:10", then 1:9", now we're getting up to 1:7". Because people are favoring longer/heavier bullets.

[azguy]

If you truly are interested in this topic read, Modern Advancements in Long Range Shooting. By Bryan Linz. I really learned a lot.