Rifling
Gaspard Kollner, a 15th century gunmaker in Vienna, is thought by many to have invented rifling. Others think it was Augustus Kotter of Nuremberg who invented rifling in about 1520. At about that same time some crossbows fired a bolt (short heavy arrow) through a tube with spiral grooving. This gave the bolt more stability in flight. Whether these were used before rifling in gun barrels is not certain.