Investigators were called to a home on the 4500 block of Salmon Street in Philadelphia's Bridesburg section just before 9 p.m. on Sunday.
Police say the teenager, along with at least one other person, were in the basement handling firearms.
One teenager fired, shooting the other in the cheek.
Investigators believe it was an accident.
The victim remains in stable condition.
Police have the person who fired the gun in custody.
Investigators are working to determine who owns the weapons.