With the new armor piercing bullets it is.
With any rifle round, above a .22, and probably with most pistol rounds other than maybe .25 and .32 ACP. Motorcycle helmets are designed for motorcycle accidents, not stopping bullets. Even military Kevlar helmets won't stop 5.56x41, 5.45x39 or 7.62x39 (M-16, newer AK's, and older AK's respectively) if the bullet hits near normal to the surface. (Perpendicular to it).