Rum, Optimum bullet stability comes from matching the bullet to the velocity and twist rate of the barrel. As a bullet travels down range, the velocity and spin of the bullet both get slower. There will be some magic point where the spin has slowed to where the bullet becomes unstable and starts to yaw. Farther down range, the bullet will break from the gyro effect and begin to tumble.
It's very common for a bullet to be stable at 100 yards and unstable at 200 yards. The first clue is an oval hole in the target vs a nice round hole. As the bullet gets even more unstable, you will see keyholes. If your target has nice round holes, the problem is not bullet stability. Test your loads at 300 yards. If you don't see oval or key holes, your bullets are stable. This is a common problem when shooting bullets that are too heavy for the twist rate. Example: Most bolt 223 guns have a 1:12 twist that will easily stabilize a 55 grain bullet but may have problems with a 60 grain or heavier.
When bullet velocity exceeds the barrel's twist rate, the bullet will jump the riflings and actually file off some of the jacket. Oddly enough, this will produce less spin and will result in a distorted bullet, thus instability. The evidence is in the bore when you clean the gun and find excessive copper along with inconsistent groups.
I've always found loading to factory velocities produces the optimum for accuracy. The factories didn't do all that research for nothing.
If you have a chronograph, try shooting a 10 round sample at 4 yards. If your max velocity spread is more than 100 fps, that will cause some strange down range performance. Also, weigh a batch of your bullets. If you find light ones, that means there is a small bubble or void inside. This will cause a whiffle ball effect and make weird groups.
I have seen the same phenomena and it defies scientific explanation. I suspect one of the following: velocity too fast, velocity too slow, or inconsistent bullets. I would also add the rate of fire to the formula. If you don't give the barrel a chance to cool down between shots, it could produce all sorts of strange effects.
