Originally Posted by Nightstar
All you have to do is just remove those limitations and I can describe the mechanism easily.
I'll try to do it within the limits imposed.
The barrel suspended between a pair of bearing races probably doesn't need any explanation. Bicycle headset races because they are available inexpensively in high quality and about the right ID.
As for the spin drive I envision a pneumatic die grinder turning a rubber wheel tensioned against the side of the suspended barrel.
The trick is timing the spool up with the shot, as well as determining what pressure and volume is required to produce the desired RPM. Perhaps separate air circuits with independent regulators and dump chambers so that timing and RPM are adjustable independent of the projectile propellent. Bolt position times the spin by triggering a discharge either mechanically or via hall effect.
Alternatively we could wind a constant force spring using a gas powered cyclone type mechanism with a release tripped by a sear in the bolt path.
I probably made myself clear as mud :/
Oh. All that crap was self explanatory. How do you make it?