Dayton gunman Connor Betts squeezed off at least 41 shots during his 30-second rampage that left nine people dead — and was supplied with enough ammo and bullet clips to commit a much worse bloodbath, cops said Monday.

“If all the magazines that we recovered from the suspect were completely full . . . including the loose rounds found on the ground near him, as well as in a backpack that he carried, he would’ve had a maximum of 250 rounds in his possession at the time,” said Dayton Police Chief Richard Biehl.

Masked and clad in a bulletproof vest, Betts, 24, opened fire with his AR-15-style rifle outside a busy stretch of bars in Dayton’s downtown Oregon District at around 1:05 a.m. Sunday, cops have said.

His first victim was his 22-year-old sister, Megan, and he was about to enter a packed bar where scores of people had run for cover when he was shot dead by cops, ending his spree just 30 seconds after it began.

But the power of his rifle and the capacity of the double-drum magazine with which it was equipped — both legally purchased — underscore how much worse the slaughter could have been, officials said.

“It is fundamentally problematic to have that level of weaponry in a civilian environment, unregulated,” said Biehl.

Investigators so far have recovered 41 spent shell casings from Betts’ rifle — as well as 65 fired by the six heroic cops who ran into harm’s way to take him out— but stressed Monday that more evidence could still be found at the scene.

Officials also clarified the attack’s injury total on Monday: Of the at least 27 survivors treated in the aftermath, 14 were shot. The rest were either trampled in the frantic flight or suffered lacerations, including from shattered glass.

Among the shot survivors is a mutual male friend of Betts and his sister. Cops have spoken with the hospitalized pal — who hasn’t been identified — but said he hasn’t been able to lift the fog on what remains the biggest mystery: Betts’ motive.

“Not close enough at all,” Biehl responded when asked if investigators were anywhere near establishing a motive.

“We have a lot of evidence still to go through.”

Despite six of the slain victims being black, Biehl said there’s no indication the attack was racially motivated.

Cops are also considering initial word that Betts had a beef with a bar on the strip after being turned away earlier in the evening, according to Biehl, although he said he couldn’t confirm “with certainty” that any such interaction had taken place.

Betts had no adult criminal record, but was twice suspended in high school for maintaining a “hit list” and a “rape list” naming classmates. He was also a singer in a band called Menstrual Munchies, a part of the local “pornogrind” music scene, which features misogynistic and violent imagery, according to Vice News.

Betts, his sister and their friend drove to the Oregon District together a short time before the attack, but Betts separated from the other two once they arrived, police have said.

It remains unclear whether he intentionally targeted his sister and their friend.

“It seems to defy believability he would shoot his own sister,” said Biehl. “But it’s also hard to believe that he didn’t recognize that was his sister.”

With Post wires