Marginal Stability: SG IS 1.0 TO 1.49. Note the box above the line graph: Your bullet is marginally stable. This does not mean that the bullet will be unstable in its flight, in fact you may be able to shoot good groups under these conditions. However, it does mean that the bullet Ballistic Coefficient (BC) will not be optimized. This information can be found on the top right area of the example above. It first shows the BC the bullet would have when stable, then what the adjusted BC would be for the given twist rate, the percentage that the bullet BC is compromised, and then the minimum twist rate we recommend for you to get optimal results with the given bullet, load, and environmental conditions.
At short ranges this effect on the BC will usually be too small to notice. At mid-ranges out to 600 yards you’ll begin to see the effects. At long range (600 yards plus) your groups will grow even more. That being said, in most cases a marginally stable bullet will not tumble and will be flying point forward to the end of its flight.
To see just how big the effect on BC can be in your case, visit the Berger Ballistics Calculator then fill in all the required parameters with your data. First, calculate your range card using the full BC of the bullet you are using. Then calculate your range card using the compromised BC value you got from the twist rate calculator. Now compare the drop at various distances. This gives you a rough idea of how much the reduction in BC will affect your shooting experience.