Bullet RPM and performance

[Grumpy old man]

If that was true it would affect the bc. Flying point up. Most of my shooting is well beyond 1000 yards yet the bullets hit where they should. I've read this too but I believe it's a myth. Every bullet I shoot is "over spun"

Every bullet isn't the same

 

[Grumpy old man]

Most of my twist rate observations have been with the 6.5 caliber cartridges using lead core bullets which I have shot most frequently over the past 20 years or so for both competition and hunting with 8.5, 8, and 7.5 test barrels. I don't buy into recent craze to automatically go with fastest twist barrels. You can't over-stabilize a bullet. Alternatively, I don't use bullets that are prone to blow-up due to over-spinning them at very high velocity. My practice is to use the slowest twist that will fully stabilize my bullets(min 150-175SG). I don't simply pick the fastest twist. From my experiences, I seem to get better and more consistent accuracy with less fouling effects at high shot counts with intermediate 8tw barrels using the 140 class, and more recently, even the 156gr bullets(that exhibit a relatively long bearing surface) which stabilize well at my velocity/conditions and perform well on game at distance. I also believe that the faster twist may be harder on the throats.

Boron nitride bullets have been very promising on faster bullet and closer groups!

 

[FEENIX]

To spin a heavy object from 0 to 240,000 rpm in 3 milliseconds means the barrel rifling is experiencing tremendous forces and very significant energy is transferred into rotational kinetic energy vs forward motion kinetic energy. However, I'm not an expert except to say the last time I analyzed this I couldn't figure out why, on paper, a bullet needs to be spun that fast.

This is why most bullet manufacturers have a ballistician or a team of ballisticians.

 

[vancewalker007]

Hopefully I am not beating a dead horse here or asking an ignorant question.

I have read about twist in relation to bullet stability and velocity in relation to bullet performance at long range. I just can't recall about reading if RPM contributes to bullet performance (i.e mushrooming and transfer of energy).

I know we have all read instances of penciling, the ELD-X bullets are one I experienced first hand and at a variety of ranges.

My questions are:

Have designers taken barrel twist/bullet RPM into consideration when designing these bullets and as twist rates get faster, ranges increase, bullets get longer and velocities get higher are these contributing factors to bullet design and performance?

Can you spin a bullet too fast to inhibit performance, increase failure or increase the chances of getting penciling?

Does spin have an affect on performance and has a single design been subject to testing at differing velocities, RPMs and ranges for terminal performance or has the data been gathered?

Is there a way to know you are in the sweet spot in relation to twist and bullet/load selection other than accuracy at the range?

I am in the process of deciding on a new barrel and other than stability for the chosen bullets, I am wondering what other effects twist will play with my selection.

Thanks Darryle

One of the most common things I've seen reported in the last 3 years have been spinning up bullets too much. A lot of people have reported bullet failures, ie. bullets breaking apart before reaching the target in fast 6.5s. The Hornady 147 ELDM seems to be vulnerable to fast twist barrels from the various mags floating around 6.5 PRC, 6.5 SAUM, 6.5 300 Weatherby, 6.5 Shermans. A lot of reports against 7-7.5 twist 4 groove barrels. I have a 1:8 on my 6.5 PRC and it has eating everything with no problems.

 

[PNWdude67]

Please elaborate; was this through a ballistic gel test? Thanks!

Think he is referring to the bullet jacket and core coming apart mid flight, usually not far down range you will see a white puff and that's all she wrote! Too fast a twist with high velocity.

 

[FEENIX]

Think he is referring to the bullet jacket and core coming apart mid flight, usually not far down range you will see a white puff and that's all she wrote! Too fast a twist with high velocity.

That's why I am asking because I shot a couple of boxes of ...

... an MV of 3602 FPS out of my 1:7" at 370K+ RPMs without any issues. SImilarly ...

I have 257BEE 7.5 twist Bartlein 26" built specifically for the 131 Gr BlackJack Ace. I have shot some 100 Gr factory ammo out of it and does very good, shoots about .75 group at 100. I have some 87 grain factories I am going to try before breaking them down for the brass, I think both 100 Gr and 87 Gr are Nosler bullets.

 

[azsugarbear]

I think the football analogy doesn't work. I think the football will stay point up not because of too much spin but because it started out point up too much. In other words, the football started out with the point higher than the plane it is traveling on.

I believe that was the point Litz was making when he pointed out that this scenario of negative effect of over-rotation was possible when the angle of the barrel (or QB's launch) was 10-20 degrees above the vertical plane.

 

[BallisticsGuy]

So, you can't over stabilize a bullet...
But you can over rpm a bullet?...cup and core that come off In flight.

That's exactly true. Stabilization is to do with the nose not trading places with the tail. Excessive rpm is about the bullet physically coming apart in free flight before it ever does any useful work.

 

[RidgeTop]

I have heard the bullet comes out of the barrel with the nose actually rotating in a cork screw fashion. Only after going some distance does it rotate on its long axis.
I have not seen bullets blow up and I think some are going in the 200,000 rpm range.

 

[Eric H]

I now use the BISONBALLISTICS.COM stability Calculator for all my new loads. If it's outside of 150-175 I won't even try it. Looking actually closer to 150. Take for instance the HH 101Gr .243 bullet; I plugged in the numbers and running it at 3100FPS it has a stability factor of 156.

 

[BallisticsGuy]

Geting old is a double-edged sword for me. It's great for knowledge accumulation. On the other hand, you often forget where the source of that information came from. Such is my plight here. I think most of us have heard about over-rotating cup and core bullets to the point that the jacket separates from the lead core soon after leaving the barrel - resulting in catastrophic bullet failure.

Somewhere I read about a secondary problem with the 'over-stabilization' a bullet. We all know that a bullet leaves the barrel on a flat plain and begins to drop immediately. To hit targets at distance, we end up compensating for this by rasing the barrel above the taget, while keeping the sights/scope on a level plain with the target, creating the illusion of an arc in terms of the path that the bullet takes to arrive on target. With correct rotation. not only do we stabilize the bullet in flight, but we also keep the nose or point of that bullet tracking the same arc. In other words, the nose leads the bullet to the target. When we over-rotate a bullet, it still stabilzes the bullet, but the nose/point of the bullet begins to no longer track the same path as the bullet. Some call this the attitude or position of the bullet as it begins its downward arc toward the target - resulting in the belly or side of the bullet leading the way to the target as it drops. When this happens, the bullet tends to hit the target a little flat or sideways. In turn, this results with the bullet not opening correctly.

I can't remember where I read this, but we have all seen similar results in a long pass or kick of a football down the field. With the correct twist to stabilize the football, it comes down nose first into the arms of the receiver. Too much spin, results with the football arriving on target, but in a 'nose-up', belly first attitude into the arms of the receiver.

If I am wrong, feel free to correct an old man. If I am right, please let me know where I read this. It will make my day and confirm for me I am not yet too far gone.

It's called angle of repose and it's a real thing but you have to keep in mind the angles involved are tiny. If you run 60moa of up you're at 1 degree of nose up attitude. 60MOA is the kind of dope you need to reach out to something like a mile with a rifle cartridge capable of getting a bullet there supersonic. I've seen ballistic effects that seem to suggest there was nutation of the bullet, actual drop curves being lumpy vs smooth predicted drop curves. At the receiving end it doesn't really matter for bullets because even if they don't expand in the textbook mushroom they're liable to fragment with some part of the tip breaking off and forming its own wound channel. That's kinda how M193 projectiles were meant to work. They don't "expand". They "fragment" which is totally different because of the spelling.

 

[BallisticsGuy]

This all begs another question from my pea brain now.

Does the amount of rotational spin affect spin drift? Does a 7 twist increase/decrease drift versus a 9 twist?

Yes. Since it's what causes it amplifying the magnitude increases the effect.

 

[Joejack]

I only got 4 rds down range today in my 22creed and it would blow up every bullet after that cleaned it it was not that bad shoot 4 then it would blow up bullets ran them over magneto speed 3210 3203fps. 26" 1&7 twist bartlin barrel 240rds on it now throat looks good

 

[Darryle]

I want to thank everyone for the education, even if it is outside of my complete comprehension.