The first known spiral grooving of a long gun barrel was done by Augustus Kotter of Nuremberg in 1520. The Germans already had a history of manufacturing crossbow bolts with their feathers offset aprox. +- 5 degrees to the line of the shaft, causing spin in flight that increased accuracy. So the base knowledge that spinning a projectile increased accuracy was available before the popularity of firearms.
The first known standard troops with rifled barrels were also German. Jägers (German for woodsman or huntsman, usually employed by landowners to provide game meat and "dissuade" poachers), were also a type of soldier, a form of light infantry, first named by the Landgrave of Hesse when he formed such a unit from his foresters and huntsmen in 1631. Huntsmen and foresters, recruited in certain German states, were long an established presence in (Preunited Germany) German military units.
Sources,
http://firearmshistory.blogspot.com/2010/05/rifling-history.html