Very uncommon. Rifles were expensive and difficult to maintain since the rifling both made it difficult to clean (by holding gunpowder residue) and wearing out from usage. Rifle units were very rare as well, you might hear about a Prussian or Austrian militia battalion that had them but aside from that only the British used rifles in "large" quantities.

Napoleon had looked at rifles once during his reign but the reload time maintenance issues, and different calibre of shot (since most rifles used smaller rounds than muskets ), it turned him away from rifles.