Much of the previous increase has been attributed to a rush to buy weapons in wake of the theater killings in Aurora, Colorado in July.
Delia said his Wissinoming store has sold about 20 AR-15s and AK-47s, two semi-automatic assault rifles, since Newtown -- nearly his entire stock.
"That wiped us out," he said. "As soon as they start talking about assault-weapons bans ... they're going off the scale."
The shooting has made gun control a hot-button issue, and high-profile debates over firearm regulations can lead to more people visiting shooting ranges and purchasing firearms, some owners in Philadelphia and across the country say.
At Delia's, nearly a dozen people were in the small shop mid-afternoon on Wednesday. When a reporter mentioned Newtown, conversation immediately turned to fears that it would soon be harder to buy firearms. Delia said his store has been bustling since Friday due to those concerns.
Employees at several other stores and shooting ranges in Philadelphia declined to be interviewed, but noted businesses has been busier than usual. Gun shop owners across the country have given similar accounts.
"Anytime there is an event like this, there is a surge to control, and there is a surge to buy before the control hits," said Yuri Zalzman, owner of The Gun Range in North Philadelphia.
Zalzman doesn't sell firearms and said it's too early to say whether Newtown and the surrounding debate about firearms will impact his business. So far, he hasn't seen backlash or wariness toward shooting.
"I had a normal Saturday, a very busy Saturday," he said. "No cancellations."
But there are signs the public and elected officials are cooling toward firearms in Newtown's wake. President Barack Obama said Wednesday he wants to send Congress a plan for tightening gun laws by January.
In Pennsylvania, Sen. Bob Casey (D., Pa.) told The Philadelphia Inquirer he is "haunted" by the Newtown killings and now supports bills to ban assault weapons and high-capacity magazines. Philadelphia officials have also called for tougher firearms laws.
Dick's Sporting Goods suspended sales of a popular semi-automatic hunting rifle. And a weekend gun buy-back program in Camden drew a record number of people handing over firearms.
There have been spikes in gun sales after other mass shootings, including the July rampage at a Colorado movie theater that killed 12, and a January 2011 attack that killed six and wounded others, including then-U.S. Rep. Gabrielle Giffords in Tucson.
In Pennsylvania, 835,293 applications came in for background checks for 2012 through the end of November. In 2011, 718,934 applications came in for the total year, according to the FBI data.
In New Jersey, 75,804 applications came in through November. In 2011, the total for the year was 60,256.
Still, some in the gun industry are calling for better controls to prevent firearms from falling into the wrong hands, such as accounting for psychological issues in background checks or eliminating the so-called "gun show loophole" that allows people to purchase a weapon from a private seller at a gun show without undergoing a background check.
"They can sell to anyone," Delia said of the loophole. "We gotta stop that."
Contact Emily Babay at 215-854-2153 or firstname.lastname@example.org. Follow her on Twitter @emilybabay.