Cannon Cocker, we determined the parameters for stabilization by using various rifling twist calculators over the past eight years. The best one we have found is:
A Calculator for Barrel Twist Rate by http://kwk.us/twist.html
Although there are many different twist rate calculators our there in Cyberland, this is the one which has proven reliable for us.
You will find that the average of the twist rates for lead, steel and zinc projectiles may hover around the 50 or 60 inches for one complete twist, what you must always remember is that in the world of small cannon twists, there is no such thing as over stabilization. So the twist which you select to make or buy must be a logical decision based on the stabilization requirements of all the projos you expect to shoot in your cannon. Because over stabilization only exists in the most highly refined, ultra long range target shooting, you must logically discount it's siren song which will lead you to the false god called "Optimum Stabilization". Your twist rates will become less quick and will attempt to provide the optimum for one length, one material and one size projectile shot at one specific velocity.
The foolishness of this will become apparent only after you begin to notice that certain projectiles at lower velocities or perhaps made of zinc are only marginally stable or stabilize only past 200 yards (this has happened to us early in our tube rifling experiments).
So, one in 42" is a low, all inclusive figure and one which was very carefully selected to give excellent accuracy in the vast variety of loads, projectiles and materials the small cannon shooter is likely to use.