I have posted this before, but it is a point to keep in mind, that the gun doesn't have to BE a machinegun to raise questions. In a case several years ago, a young man installed one of those dummy selectors on an M1A. (He later said that he just wanted to "needle the pigs". He did.) A police officer saw the gun, concluded it was just like the M14 he had carried in Vietnam, and took the man in for questioning. He did not make an arrest, which turned out to be good. The man kept telling them it was legal, but strangely, cops don't believe protestations of innocence. There is no doubt that "probable cause" was there - the gun looked like a machinegun, and the man had no papers. (Federal papers may not have to be with a machinegun, but MD state registration papers do.)
They got the police firearms identification expert off the golf course on Sunday and, even though unhappy, he told them the gun was legal and the man was released. But he came close to having an arrest record over nothing more than his juvenile desire to stick it to the cops.