1 in 40 cm is pretty close to 1 in 16" twist which sounds a lot closer to what will work. I would be interested in the results of your project. Good luck and have lots of fun.

Correct twist rate is paramount to accuracy. 1in 8" twist sounds pretty fast for a projectile that light weight and for that length. I`m working on a rifled .32 barrel with a 1 in 16" twist and 700 fps and am getting a length of projectile of .538. I`m going at this from the other direction of ...

Twist rate depends on velocity and length and caliber of projectile. Without knowing these there is no way to calculate twist. Most small caliber air rifles are around 1 in 16" rifling at around 700 fps. Longer projectiles need a faster twist than shorter. Twist rate is not weight dependent. Also ...