'might' & 'some'. Words I always prefer, along with 'maybe', in my facts.

Jeff_Schultz wrote on Jan 15^th, 2014 at 4:43pm:


I rely on the work of Ruprecht Nennstiel.

Regarding gyroscopic stability: As the spin rate decreases more slowly than the velocity, the gyroscopic stability factor sg, at least close to the muzzle, continuously increases. Thus, if a bullet is gyroscopically stable at the muzzle, it will be gyroscopically stable for the rest of its flight.

See the graph at (You need to Login or Register to view media files and links)

Regarding dynamic stability:  A bullet is said to be dynamically stable, if an angle of yaw, induced at the muzzle, is damped out with time, or in other words if the angle of yaw decreases as the bullet travels on. 

There is no corresponding graph for dynamic stability.

A projectile is least stable (both gyroscopically and dynamically) at the instant it leaves the crown.

JackHughs

When a bullet is fired and goes down range it goes through at least two and sometimes three moments of instability. The first is when it transitions from the barrel to the air. The second happens at the moment when the bullet passes through the sound barrier. The third is happens when, if you are shooting in that range, the bullet slows down enough to pass through the sound barrier again. 

40 Rod

So we all know the crown is critical, something off will throw the bullet off, nice crown has proven to correct problems.  Now we introduce something else into the equation. Tube around the muzzle, bullet leaves the well cut crown and subject to all sorts of potential problems with captured gas working on the bullet. Is the Bloop tube on straight, does it add vibration to the barrel ? Wind at muzzle you can handle by picking the condition and not shooting with adverse wind, traditional way to manage the variable.  Tube is a mechanical way to attempt to manage wind, may work may not work.

Seems to me bloop tube is a variable that can be ruled out by not using one.

Boats

marlinguy wrote on Jan 17^th, 2014 at 11:24am:


Force = Mass times acceleration.  Or, put another way,  Acceleration = Force divided by Mass.

While the projectile remains in the barrel and if combustion is not complete, a propelling Force is imparted to the projectile by the expanding gas.  Once the projectile leaves the barrel, the propelling force drops to zero and the projectile's forward Acceleration also drops to zero.

JackHughs

Many years ago I saw a series of films taken at very high speed using a special camera which clearly showed that bullets emerging from the barrel did have a definite wobble during the first few feet of flight, where after the bullet stabilised into normal flight. The sound accompaniment said that the rounds were standard 30.06 military issue, so they would be emerging from the barrel at supersonic speed. 

What I believe causes the initial instability is the fact that the shock waves generated in the barrel as the bullet goes supersonic have nowhere to go and build up in front of the bullet, these are released as soon as the bullet passes the crown, but until they have dissipated and the shock wave becomes stable, its only then the bullets flight settles down. In my opinion these compressed shockwaves could be the cause of the initial instability. 

Harry

JLouis wrote on Jan 18^th, 2014 at 12:12pm:


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

Quote:

 
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