Can anyone tell me >>> Why is it that all the 300 BO folks require a 1:7 twist to stabilize a sub-sonic 220 grain .308 bullet and yet my Bergara B-14 HMR can stabilize the same bullet at the same MV with a 1:10 twist barrel (at sea level no less!)???

What caliber is the Bergara?

The heavier (for the same caliber) a bullet is the faster it needs to spin to be stabilized at lower velocities.

If the .300 BO could push a 220 at the velocity a .30-06 does it could be a 1 turn in 10" barrel.

.223's used to be loaded with 55 grain (max weight bullets) and had barrel twists of 1 turn in 12" (some even 1 turn in 14").  Along came heavier bullets for the .223 and barrel twist went to 1 turn in 9".  Then even heavier bullets came along and barrel twist went to 1 turn in 8" or 1 turn in 7".  As the bullet weight went up, the bullet got longer and the velocity got lower and the bullet had to have a faster spin barrel to maintain stability.

Q has been using 1:5 on their 300 Blackouts.

The rifle is chambered in .308 Win but I am loading 220 grain bullets to leave the barrel just below the speed of sound...

I've heard that... makes even less sense to me!!!

I don't understand that idea.  Barrel length is not an input parameter in any of the equations I have seen to calculate stability?!?!?  My understanding is that RPMs are what stabilizes a bullet.  You can calculate RPMs based on MV and twist rate...  How many RPMs are necessary to stabilize any given bullet is based on some of the ballistic characteristics of the bullet and some atmospheric conditions...  What am I missing here???

I DO ADMIT that a 220 grain .308 bullet is marginally stable ("by the numbers") at sub-sonic speeds when fired from a 1:10 twist but 1:9 is plenty of twist for 220s and nobody uses it.  Manufacturers even shy away from 1:8 twist in favor of 1:7...

Intuitively, that would seem to make sense but there seems to be no math backing up that "intuition"...  Also, I have a .357 mag derringer with a 2 inch barrel...  I'm pretty sure it doesn't have a 1:2 twist... but the bullet holes in the paper are all nice and round!...  not very close together mind you, but definitely nice and round nonetheless!!!  

Ok... I'll try to do this so that it makes sense and is not overtly mathematical and avoid technical terms. This necessitates losing some precision in the description but the idea should shine through.

The bullet is pushing through the air nose first and so there's pressure on the nose that's much greater than the pressure everywhere else on the bullet's surface. If enough pressure is put on the nose then the nose will not be able to maintain its position in front anymore, it will be moved from its position dead in front and the bullet will tumble. The whole reason for rifling existing is to keep that from happening. Spinning the bullet creates what's called a "moment arm" that is longer the faster the rate of bullet spin. That the moment arm gets longer the faster the rate of bullet spin in essence changes the numbers in the equation to be what it would if the bullet were a lot shorter and fatter than it is. Shorter fatter bullets take less spin than longer skinnier bullets to be stable. The more pressure on the nose compared to pressure everywhere else on the bullet, the faster it needs to spin to not flip over. The longer the bullet, the faster it has to spin to not flip over. Stability is that point where there's enough spin to keep the nose in front despite all the forces trying to make it go somewhere else.

.300BO has a special problem that is not encountered commonly by a lot of other chamberings. It's got a stupid long heavy bullet running just under Mach 1 at the muzzle. Well, within a couple hundred feet per second of Mach 1 is where a bullet encounters MAXIMUM DYNAMIC PRESSURE. This is where the pressure on the bullet nose is greatest and it's smack ass in the sweet spot of 300BO muzzle velocities. Yep, at 1000fps the bullet nose is being pushed on by air harder than it is at 1300fps. The speed of sound is a strange mistress. 300BO velocities being so low with heavies means that if you go to a bullet that's even a little bit longer than one that worked fine before, you may find it doesn't work now so a lot of .300BO's come with insanely fast twists so you can stabilize a 230gr bullet at 800fps.

What you're looking for with modern pointy bullets is an Sg of at least 1.3-1.5. The lower side of that reduces ballistic coefficient, much more than the high side is a waste. What is NEEDED by a .308 220gr bullet at 900fps is just a little more than 10 twist just so it won't come out of the muzzle an immediately tumble. At 1:9 probably won't keyhole. At 1:8 it definitely won't keyhole. At 1:7 it's super dee duper stable.

Colin;

Thank you for the wisdom!  Yes indeed, the speed of sound is strange mistress ballistically speaking!

My AO is at sea level but temps are typically pretty warm so DAs for me are anywhere from 0 to 2,000 ft.  I have shot a variety of subs (900 to 1,050 fps) out of my 10 twist .308 including Nosler CC 220s that are 1.52" OAL and have never seen keyholing in any targets out to 300 yards.  Perhaps not super dee duper but seems workable!...

Thanx again Colin!  

- M -

Exacty true. You can get by with a 1.0 Sg, they'll lose a ton of BC because the nose will be precessing in something resembling epicycles (think aristotelian model of the planets) which radically increases drag. If you're at 10 twist pushing 220's then your Sg at 1000fps is about 1.05 which is just barely on the right side of stable. As velocity decreases the bullet will become more stable so unless they keyhole pretty much right out of the muzzle, they likely won't since you're starting on the far side of the transonic zone already.

The Bernoulli equations break down when the Mach wave forms in supersonic flight.

Bernoulli assumes air is incompressible.
At the speed of sound air stops acting as an incompressible media.

The 'flow' equations must be rewritten.
Standing pressure waves significantly alter air flow over a surface
they are attached to.