Actually, enforcement in the UK and Australia has been pathetic, with around 20% compliance rates.
You can't legally buy a firearm in the UK without a firearms certificate. When you buy the firearm from a dealer the proof of purchase has to be sent with the firearms certificate and number of the firearm to the police firearms unit for approval. If its a handgun then it goes for a ballistic test that will be kept so a bullet can be traced back to the firearm. So all legally bought firearms are registered. So if you are caught with a unregistered firearm there is a good chance you will be going to jail.
Quote If found guilty the penalty for possession of a prohibited firearm without a certificate is a maximum of ten years in prison and an uncapped fine. Unauthorised possession of most kinds of firearm attract a mandatory minimum of five years.
PS I know the thought of having similar checks in America would not go down well to put it mildly.