Punk picked wrong gran!

A pistol-packing Harlem granny turned the tables on a robber yesterday, busting out her registered .357 magnum and shooting the mugger in the elbow - while riding in her motorized scooter, cops said.

Feisty Margaret Johnson, 57, who has a dislocated hip and a herniated disk, was heading out for target practice about 3 p.m. when a career criminal came up behind her and went for her necklaces, sources said.

"There's not much to it," she said later. "Somebody tried to mug me, and I shot him."

The thief "walked past her, and came around and choked her," said Johnson's pal, Lynnell Bunce, 40. "He was going for her jewelry, and he got it. He didn't say anything - until she shot him."

Then the man cursed and fled from the scene outside 470 Lenox Ave., where the victim has lived for decades, Bunce said.

"I'm very tired. It was very scary," Johnson said as she later entered her building in her three-wheeled scooter with scratches on her neck and left hand. "I've had a very long day."

Police followed the trail of the wounded man's blood to a McDonald's restaurant not far from Johnson's building and arrested 45-year-old Deron Johnson, an ex-con with nine robbery arrests who is no relation to the victim, sources said.

He was in stable condition at Harlem Hospital last night, and charges were pending against him. The jewelry was recovered, officials said.

Margaret Johnson has a premises permit for the gun, a police source said, meaning she is allowed to keep the pistol in her home and transport it to a firing range.

Bunce said her friend, who has a grown son and at least one grandchild, got the permit when she was a bus driver for the city.

Neighbors said Johnson, a granddaughter of a famed Harlem gangster, the late Ellsworth (Bumpy) Johnson, is a well-known and well-liked woman.

"Everyone loves Margaret," Bunce said. "Margaret is funny. She's like clockwork. She's lived here forever, and she walks her dog four times a day and says hello to everybody."

"She's spunky," said Toi Frederick, 26, a lawyer who lives in the same building. "I wouldn't think she would be someone who would just lay down and give up her stuff."