A true barrel roll requires a constant rate of pitch and roll throughout the manoeuvre which helicopters cannot achieve as they run out of speed. Most rolls in helicopter aerobatics are the equivalent of an aileron roll in a fixed wing - that is to say they are a product of lateral cyclic. The ability to achieve a rapid roll rate is required so hingless or semi rigid rotors have the advantage here due to increased effective hinge offset and therefore control power. The problem with snap rolling a helicopter is the twisting force experienced by the tail boom - the mass of the TR, TRgearbox and drive shafts has a lot of inertia.
The Lynx display manoeuvre called the Eagle roll starts with a gentle nose up to reduce the speed followed by full right cyclic and a lot of right pedal. The result is spectacular both from inside and outside the aircraft but you pay the penalty of vastly reduced component times and increased servicing for the pleasure.