Bullet RPM and Drag — How BC Changes with Bullet Spin Rates

[Elric]

Bullet RPM and Drag — How BC Changes with Bullet Spin Rates « Daily Bulletin

You may not realize it... but to get the optimum BC from your bullets (i.e. the lowest aerodynamic drag), you must spin the bullets fast enough. Bullet drag increases (as expressed by lower BC) if the bullet spins too slowly.

bulletin.accurateshooter.com

"Barrel twist rate along with velocity, atmospherics, and bullet design all combine to result in a Gyroscopic Stability Factor (SG). It’s the SG that actually correlates to BC. The testing revealed that if you get SG above 1.5, the BC may improve slightly with faster twist (higher SG), but it’s very difficult to see. However, BC drops off very quickly for SGs below 1.5. This can be seen in the figure below from Modern Advancements in Long Range Shooting. "

Interesting, though some cast bullets (pistol) don't do better than a BC of .1, some rifle bullets do much better, maybe .3 or so IIRC...

 

[Ian]

Darn, I'll have to get rid of all my 17-twist 30 caliber barrels now.
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Oh, wait, I never got rid of my 10-and 12-twist barrels in the first place because I never needed to. If you're gonna shoot cast at jax velocities you'd better realize that they're subject to the same laws of physics and yeah, some smart people a long time ago figured out how much spin is needed and sort of standardized twist rates to match.

 

[Elric]

I am coming up empty when thinking of any cast bullet load that I have jerked the loading press handle on that did better than, say 1600 fps if that...

As I have achieved enlightenment, so can others...

Cast bullets don't need to be driven hard in order to expand. Just need the correct alloy. Or... if you need to drill something big n nasty, a good hard alloy will help penetration...

Caveat: I am not a professional hunter from the dark continent, nor do I moonlight as a Grizz hunter from Alaska. If you intend to hunt things that will bite, tusk, or claw you if they aren't DRT, check with recognized experts in the field....

 

[Spindrift]

Interesting reading, thanks for posting!
The thing with cast bullets is, they have higher density than jacketed bullets since they are unpolluted by the lightweight jacket material . Which means a cast bullet of a given weight is much shorter (and hence, inherently stable) than a jacketed bullet.
I have used the Berger stability calculator once, when preparing the NOE 311284- clone (220grs) for subsonic loads in a 1:10 .30-06. I had anticipated stability problems. But; I got an SG of 2,5 (or something), round holes at 100m and decent accuracy.

 

[Elric]

Why I love my 285448

 

[Ian]

I've shot subsonic 247s in a 12" twist and done well enough. The only stability problems I had was testing the limits of .35 caliber for our friend JWP, I found 220-grain .35s in a 16-twist need at least 1300 fps to go straight on a shirtsleeve day at 2,000 ft elevation, any slower and they start to wobble badly. I put the Lee 309-230 into tiny groups all day long with a 10-twist .308 at 1,000 fps MV, no issues at all.

I also found if you put a boat tail on your cast bullet, they take a lot more range for the yaw to dampen, like more than 100 yards. Same thing if the bullet has its CG very close to its CP, like under 2% difference, as in the 6.5mm x 170-grain Lee bullet which is nose-heavy and just about dead-even with regard to CG/CP. Deep hollow point bullets help a lot in this respect.

 

[Elric]

I also found if you put a boat tail on your cast bullet, they take a lot more range for the yaw to dampen, like more than 100 yards. Same thing if the bullet has its CG very close to its CP, like under 2% difference, as in the 6.5mm x 170-grain Lee bullet which is nose-heavy and just about dead-even with regard to CG/CP. Deep hollow point bullets help a lot in this respect.

So, order of time to go to sleep - longest, boat tail, gas check, plain base... A modifier is the existence of a hollow point, which shoves the CG back... or something like that...

 

[RBHarter]

Oh Lord it's can o worms .


I've read many entries about twist relative lengths and done enough rooting around trying to get to a number from steel shot , copper what ifs , and bullets in calibers and cartridges that are just right at the edge of making it or not .
In real done it applied work .
I had a 1-12 30-30 I only had the 311-230 that wouldn't shoot .
I had a 1-12 308 that wouldn't shoot over 175 gr . I don't believe a copper plated tungsten wad cutter at 178 gr would have shot in it .
I had a couple 1-10s in 06's never could get a useful cast over about 2000 fps .
The last 06 was 1-8.5 I suspect a special order intended for the long time out of production 225-250 gr spitzers that faded away in the 1970s only to come back recently for the 300 BO . It and the SKS I had wouldn't do but 1800 with anything that would expand at all .

Having had 22s in 1-14 and 1-8 , the 1-14 let's a 62 gr version of the 22-55 run 2640 but the 1-8 is only good to 2050 .

35s I haven't explored the limits with different bullets .

A 1-32 will stablize a Lee 340 but not my custom 453-350 .
Supersonics destabilize in transition and take off like a curve ball .
Appearently a 1-20 will manage a bullet 1.6" long from 900fps to 1600 . I only tried loads up to about 1200 fps . I'm thinking I should be able to get 22-2400 , I got 1800+ with a 250 in a 1-16 .

A 1-10 in 6.8 SPCII was heavily depending on powder curves , 279-124 NOE in a 16" was over pressure and under gassed I guess but would go 2100 on 4198 but would cycle clean on H322 .
Moving to a 1-11 20" the 4198 loads went 2300 and clean cycle but the H322 loads gained only 100 fps . I don't think I hit the top I think I ran out of powder curves and barrel length .
A 7x6.8 paper patched 27-130 @141 gr was driven 2350 with 1 MOA groups in a 9.5 twist just a little and I poofed primers . I need to shoot some naked 110 or 120s in both the 6.8 and 7×6.8 just to see where the failures happen . In the 7mm a 287-150 will run Straight from 950 fps up . I can't get the 7mm168 to cooperate , not enough nose dia , I don't think I can hit those hard enough to reach spin failure ....1-20 maybe .

 

[Will]

As long as I don’t have to read the word “Threshold” I think I can stand to read all this.

There are many threads on 308’s with 1-10” twist shooting 2300+FPS.

 

[Intheshop]

"Threshold" and "Hardcast" have probably moved the sport backwards more than any other two words?

Slow twists go great with short range BR and also,light varmint bullets,nuff said for me.