The US gun industry is trying to shake off the Hollywood hit man image of the gun silencer and rebrand it as a hearing-protection device in a campaign to roll back regulations that date to the 1930s. Industry lobbying has led to more than a dozen states legalising silencers for hunting since 2011. Now gun advocates are pressing Congress to repeal a Depression-era law that requires a months-long screening process for silencer buyers - far more scrutiny than gun buyers face.Sales of silencers - or "suppressors", as the industry prefers to call them - are booming. The number of silencers registered with the US government more than doubled to 792282 in February last year from 360534 in March 2012, according to the US Bureau of Alcohol, Tobacco, Firearms and Explosives.Despite their name, silencers can only quiet a gunshot to the level of a jackhammer - not much use for James Bond-style hit jobs.Silencers are rarely used in crimes, according to a 10-year study published in 2007 by the Western Criminology Review.Researchers estimated silencers were involved in 30 to 40 of the 75000 federal criminal cases filed each year. Arizona Republican representative Matt Salmon said silencers could allow soldiers and hunters to avoid the kind of hearing damage that has forced him to wear a hearing aid after 50 years of shooting guns.In October, he introduced legislation that would replace the silencer screening process with a simpler background check...