The rifle will be raffled May 13.
Welsh said that as "the mother of four and the grandmother of nine," she understands residents' concerns.
But while some have called for a ban on the AR-15 and similar firearms, the weapon is still legal. However, the winner will undergo a background check before claiming the prize, Welsh said.
That is heartening, said West Chester Mayor Carolyn Comitta. Under Pennsylvania law, people who buy long guns such as rifles and shotguns in a private sale are technically not required to undergo a background check. Walsh will conduct one anyway.
"What I choose to focus on is the fact that she is requiring a background check and that this is certainly a good model for all gun sales in Pennsylvania," said Comitta, a member of Mayors Against Illegal Guns.
Still, following mass shootings such as the ones at Sandy Hook, raffling an AR-15 seems "a little insensitive and tone-deaf," said Shira Goodman, executive director of the gun-control group CeaseFire PA.
"Especially when this is such a superheated issue, and people are getting engaged and trying to find common ground and move forward, maybe this was not the best decision," Goodman said.
Welsh said she was committed to encouraging responsible gun ownership and gun safety, and added that the raffle, though open to the public, was geared toward hunters and law enforcement personnel.
"We were never out publicly selling tickets," she said. "The sale was to sportsmen, hunters, and law enforcement, and that will be the type of individual that will acquire this weapon."
Funds raised will go toward training for the sheriff's K-9 program, which includes dogs trained in explosives, narcotics, and accelerant detection.
Contact Aubrey Whelan at 610-313-8112, at firstname.lastname@example.org or on Twitter at @aubreyjwhelan.