Critics contend that background checks are unnecessary because long guns are rarely used in crimes.
According to FBI data, rifles and shotguns were used in at least 8 percent of gun-related murders from 2008 to 2012. And long guns were used in 21 percent of domestic-violence shooting deaths, according to the FBI.
Appearing at the news conference, Philadelphia Police Officer Rick Bowes said long guns were disproportionately used in shootings of law-enforcement officers.
His partner, Sgt. Patrick McDonald, was killed in 2008 by a parolee who had illegally purchased a gun. The shooter also hit Bowes twice before the officer was able to return fire, he said.
"Law enforcement, we walk around with a target on our backs everywhere. We're outgunned out there. These long rifles - we don't have the ammunition to come close to that," Bowes said. "Little loopholes like this that we can close makes it safer for us."
The bill has more than 50 co-sponsors, including four Republicans, said lead sponsor Steve Santarsiero (D., Bucks). Also at Thursday's event were Reps. Madeleine Dean and Matt Bradford (both D., Montgomery).
"House Bill 1010 would close the last remaining loophole in our background check system to ensure that criminals and people with mental illness do not get ahold of guns," said Santarsiero. "That's all it does."
Rep. Daryl Metcalfe (R., Butler), a strong opponent of gun-control measures, said the bill is "a solution in search of a problem."
"It wouldn't close a loophole, it would create an additional burden on the already overloaded check system, for firearms that have not been a problem in crimes," Metcalfe said.
Rep. Paul Clymer (R., Bucks) also said he would oppose the bill. "This is, again, putting more government on the law-abiding citizen who uses firearms in a very careful way, if they use them at all," he said.
If it passes, private buyers would have to visit a licensed gun dealer or sheriff's office to complete the background check. A similar process is already in place for private handgun purchases.
Transfers between immediate family members would still be exempt from the background check.