FrankenMauser
New member
Velocity and twist rate go hand-in-hand.Road_Clam said:
Ideally, one increases as the other decreases. That's why cartridges like .220 Swift (high velocity) run 'slow' twist rates, while lower velocity cartridges (like .223 Rem) run faster twist rates.
All you have to do to improve the stability of a 'marginally-unstable' bullet is bump up the velocity. (If there's room to safely do so.)
If the velocity used for a calculation differs from that of the actual load, then you're almost certain to get a different result.
And, there are almost always other factors at work, that can have a dramatic impact on bullet stability and the accuracy of stability predictions.
Have a lot of free-bore that tends to cause bullets to go down the barrel slightly skewed? You can bet your bottom dollar that it's going to cause stability/performance issues for marginal bullets.
Have a burr on the crown? Calculations/predictions can't take that into account.
Have a crown that's not square? Same thing.
And more...
I have a 5.56x45mm-chambered rifle that was giving me fits with its 1:9" barrel. I could not figure out why its performance was so unpredictable, but predictably terrible with bullets that should have been in the 'marginally unstable' category. Eventually (after many other attempts to fix it), I broke down and rented a crown refacing tool. While waiting for it to show up, I clamped the barrel to clean and square it up for a good starting point.
And that's when the problem became clear: The muzzle had been cut about 10 degrees off of square, but the crown was still cut square to the bore.
So, bullets were being destabilized at the muzzle. Some bullets could recover (primarily flat base bullets). Others (primarily boat tails) just went nuts and printed shotgun patterns - up to and including key-holing.