Quote:[quote author=614443457467445E42582B0 link=1389546247/42#42 date=1390065162]
I have to ask Mr Louis why should the bullet be unstable when exiting the barrel if becomes stable in flight after a few yards? There has to be a reason for that, hence my hypothesis of supersonic shock waves being unable to form correctly whilst the bullet is in the barrel. It is a well known aerodynamic problem that led to many of the early jet aircraft becoming unstable or even breaking up due to compressibility as they approached Mach one.
Harry
Hello Harry,
As John said, initial yaw is largely a function of bullet imbalance. Within the barrel, a projectile is constrained to rotate about its "center of form". Once the projectile leaves the barrel, it is free to rotate about its "center of mass".
If the center of mass does not lie exactly on the center of form, the projectile is unbalanced. An unbalanced projectile will diverge from the axis of the barrel at a "deviation angle" (yaw) that is a function of the radius of gyration (the distance between the center of form and the center of mass) and the twist rate of the barrel.
As an aside, an imperfect bore and/or an imperfect chamber can cause a perfectly balanced projectile to become unbalanced as the projectile moves through the bore.
Given a perfect crown, this deviation angle is the largest contributor to initial yaw. If the crown is less than perfect, it will cause uneven force on the base of the projectile as the projectile leaves the muzzle. These uneven forces will also contribute to initial yaw.
Because of initial yaw, a projectile is strongly influenced by external forces (such as wind) as it leaves the muzzle. The effects you mention are in the class of external forces. These forces can seriously affect downrange performance and are not to be taken lightly. However, they are not the cause of initial yaw.
JackHughs