There are several type of Class 3 items that are restricted by the National Firearms Act.
Each state can impose additional restrictions on the sale, purchase, and transfer of class 3 firearms in addition to the compliance that is required with the national Firearms Act.
In Pennsylvania you can own the following items that are regulated the the National Firearms Act
Any Other Weapon (AOW)
Destructive Devices (DD) (except bombs)
Short Barreled Shotguns (SBS)
Short Barreled Rifles (SBR)
In Pennsylvania you cannot own the following NFA restricted items.
None but all Bombs are prohibited as Offensive Weapons
Follow this link to find out more about Pennsylvania and NFA restrictions on Class 3 Firearms