The Trump administration is scrapping more than three dozen firearms regulations, abandoning a crackdown on illegal sales, restoring gun rights to some people with mental illness and loosening oversight of private weapons transactions.

The drastic retrenchment at the Bureau of Alcohol, Tobacco, Firearms and Explosives, the federal agency responsible for enforcing the nation’s gun laws, was not entirely unexpected: President Trump campaigned as a champion of gun rights.

In the view of critics and even some A.T.F. veterans, the agency, in closely mirroring the demands made by gun owners and manufacturers to lighten their regulatory burden, is enacting changes at the expense of public safety. The moves, they worry, come as the bureau has already been weakened, with hundreds of its officials diverted to immigration enforcement.