Felix
A tire is not a good example to compare to a bullet. For one thing you know that a tire is not round at high speeds. It is amazing that a tire balances as good as it does considering how out of round it is at high speed. Could this be because of the gyroscopic effect making things work?
Sundog mentioneed boattail bullets so I copied the post on them by Chris Byrnes. Larry pay attention as I tell you again I copy this. By the way if there were spelling errors in that very first copy of Chris's work they are his not mine. Not to say that I do not make spelling errors as it is obvious that I do.
Here it is:
Boattail bullet stability, and the bearing surface myth.
Some folks believe that boattail bullets are more stable, and will stabilize at lower twist rates; for varying reasons, but often because they believe that the length of surface contacting the bore makes a difference in stability.
Unfortunately, this is entirely incorrect. Sometimes boattail bullets are more stable, especially at extreme ranges; and they are certainly more aerodynamic in general; but not all boat tail bullets are more stable in all loadings or situations, and if they are more stable, it has nothing to do with the length of the bearing surface of the bullet.
The reason why boattail bullets sometimes stabilize better, isn’t because of surface contact with the bore. Why would twist rate or stabilization have anything to do with surface contact with the bore?
The primary reason boattails can sometimes be more stable, is because the boattail base is less susceptible to aerodynamic upset; and is also less likely to be nicked by a rough crown or bore imperfection. Additionally, what imperfections there are have less effect on the bullet because they are at a smaller radius (and thus exhibit lower gyroscopic forces), and have less aerodynamic pressure applied to them
Often however, very light boat tail bullets are MORE susceptible to upset, because under some circumstances they do not exhibit as much positive stability (the tendency to damp out oscillations from a disturbance) as flat based bullets, as they have less incident flat plate area behind the center of pressure.
Hollow base bullets are even more interesting, in the effects that they have on turbulent flow, and base upset; as well as their deformation characteristics out of the bore which can cause a shuttlecock effect; but that's neither here nor there.
Some folks believe the bearing surface length is important, without regard to bullet design, but this too is incorrect.
You’ll note that the reason why twist rate increases stability, is because faster rotation means more aerodynamic forces (radially symmetric lift) as well as more gyroscopic forces. This has nothing to do with bearing area. Let me say this again: The bearing area and rotational velocity of the bullet in flight, are not interrelated.
You could have nothing more than a bearing band 1/8” long, and the bullet would leave the barrel spinning at the same rate, and with the same rotational energy (though a slightly higher linear velocity presuming gas seal integrity was maintained) as if the bullet were a perfect cylinder (presuming the rotational forces weren't so great that they distorted the bearing band of course).
In fact bearing bands are used by many large diameter (.50cal and above) rifled projectiles; as well as many projectiles used in 18th and 19th century in muzzle loaders; because the use of bearing bands reduces friction and fouling. Also some cast lead revolver bullets use a bearing band design for the same reasons.
Heck, boat tail bullets are even longer per given weight than flat base bullets are, so they would have an even faster required spin rate if it weren't for the factors I spoke of above. If you can shoot heavier boat tail bullets through a slower twist barrel accurately, it is for the reasons I mentioned above... welllll and one more thing
Now, I said up near the beginning of this piece, that form lift, or rather to be more specific aerodynamic characteristics based on shape, could be important; but it wasn't relevant to the discussion here specifically with regard to twist rate, length, weight, and stabilization.
That's not strictly true; because different shapes of bullet, even with identical weight and length, will have different aerodynamic characteristics based on their shape. It's just that for the most part, those aerodynamic differences are quite small between different spitzer bullet designs, different roundnose bullet designs, different flat base bullet designs, and different boat tail bullet designs.
Most often, bullets compared with each other will all be of a single design type i.e. they will all be pointed nose or all be round nose etc... There can be significant aerodynamic differences between round and pointed nose bullets, and flat base vs. boattailed bullets.
Some shapes exhibit greater tendencies toward positive stability. Some shapes have greater stability at lower velocities, and less stability at higher velocities or vice verse. Some shapes deal with things like shock waves, turbulence, or flow separation better than others.
This relates to boattail bullets specifically, because overall, the boattail spitzer (meaning pointed at the front with a gentle ogive, and tapered at the back with a flat base behind the taper) is the optimal form for maintaining both stability and aerodynamic efficiency in the velocity ranges we are dealing with for rifle bullets. Sharply pointed bullets with a tapered base generally deal with shockwaves and moving centers of pressure better than other bullet designs; resisting shockwave upset and turbulent flow upset better.
So yes, often boattail bullets will be more stable; but that doesn't necessarily mean they will stabilize at a lower twist rate (though sometimes it does). It really depends on the relationship between the CG and the CP of the bullet, as described above, total aerodynamic forces, shockwave characteristics etc... and it's not something that can be easily predicted by a lay person, or by anyone without a supersonic wind tunnel and measuring equipment for that matter.
Simply, other than the general differences between pointed and flat or rounded nose; and flat or tapered base bullets; those comparisons are not useful for practical purposes.
Ralf