Thread: BC and spin rate

I was listening to a Podcast today, Long Range Pursuit EP 127 Meet the new 7PRC.

If I understood them correctly, at one point they were saying the BC of a bullet changes if you spin it faster.

My interpretation was the same bullet has a higher BC if fired from a 1 in 9 twist vs a 1 in 12 twist.

Is this correct? I have never heard of this before, and can anyone link me to further reading or information on this?

Maybe this is common knowledge that I missed but then wouldn't ballistic calculators require the spin of your barrel?

Yes it does, and yes they do require the twist imput. Nosler are famous for having lower than advertised bc's. Impart due to them using far tighter twists than are standard or usual and deriving their bc's from them. Problem is they don't actually make apparent what they used and thus when Joe average uses his/her standard/ common twist. The bc is way off.
Speed also makes a difference. You need a lot of bc too make up for a small increase in speed. So to get the maximum bc a projectile is capable of, you need correct twist and appropriate speed.
Hence a lower bc projectile will out perform a higher bc one if it has the right speed and twist , if the higher bc one isn't spun or pushed fast enough.

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Hornady publish BC's for different spin rates for some of their bullets. Not only does it change with spin, it changes with speed too. https://www.hornady.com/support/ballistic-coefficient.

I wouldn't assume there's any clear, predictable relationship between increased speed/spin and increased BC either and each bullet could well be unique. That would make your app asking for the spin rate a bit pointless if it couldn't factor it in accurately. I use Strelok and it does ask but best I can tell that's just so it can tell you the estimated stability factor. The BC itself sometimes needs a bit of manual truing up at distance.

Thanks, learn something new every day I guess. I knew faster spin stabilized heavier bullets but never imagined it would alter the BC.

I suppose they do have a recommended twist rate for each bullet in the specs so you would assume that is the twist that gives them the best BC.

pretty sure some of the newer projectiles have been given different rates depending on speed etc.....

If it’s properly stabilized, a gyroscopic stability factor (SG) over 1.5 there won’t be much difference. https://bulletin.accurateshooter.com...stabilization/

It might also be useful to remember that as bullet speed drops gyroscopic stability does as well

Originally Posted by Marty Henry

It might also be useful to remember that as bullet speed drops gyroscopic stability does as well

If you mean muzzle velocity, then yes. If you’re talking about bullet speed during flight then SG increases as velocity drops because velocity decays faster than rpm.