The outspoken White House hopeful went on to accuse Democrat rival Hillary Clinton of wanting to weaken gun rights.
The presumptive nominee picked up the official endorsement of the powerful National Rifle Association (NRA), which boats more than four million members.
And his comments at the NRA’s national convention yesterday could make him a favourite for conservatives who see protecting their gun rights as a top priority.
Clinton, who is on the cusp of winning the Democratic nomination, has vowed to beat back the gun lobby and expand gun control measures.
But Trump claimed she wants to “abolish” the Second Amendment, which gives Americans the right to keep and bear arms.
He said: “Hillary Clinton wants to abolish the Second Amendment, not change it; she wants to abolish it.”
But an official from Mrs Clinton’s campaign dismissed Trump’s comments, branding them “conspiracy theories”.
She said Clinton believes there are “common-sense steps to keep guns out of the hands of criminals” while protecting the Second Amendment.
The New York billionaire’s NRA speech was another step in his drive to make more conservatives comfortable with his candidacy.
Many conservatives – including supporters of Ted Cruz, who quit the race last month – worry Trump secretly holds liberal views on many issues.
But Chris Cox of NRA’s Institute for Legislative Action said it was time for them to shrug off their concerns and back Trump.
Mr Cox said: “If your preferred candidate is out of the race, it’s time to get over it.
Are there valid arguments in favour of some over others? Sure. Will any of it matter if Hillary wins in November? Not one bit.”