A rifled barrel was a firearm barrel with internal surface spiral grooves that allowed a projectile to be shot further and straighter than barrels without rifling.
In 2267, the Klingon Empire contacted the natives of Neural and began giving them weapons such as flintlock rifles. In 2268, the Klingon Krell told the Neural native Apella that he would give him improvements to his rifles, specifically a rifled barrel. (TOS: "A Private Little War")