Is a tube at the end of a rifle illegal or considered a silencer?

In trying fun things to do, a friend of mine put an open ended tube about 1 1/4 inch diameter at the end of a .22 rifle. It sounds like an m203 now but I've been told it's illegal. There are no baffles no end caps no sound absorbing material, and resembles an m16 golf ball launcher. I've been told by a local nfa dealer that it's still illegal but I don't see how