I think some people are confused by Pennsylvania’s UFA in regards to how it defines firearms, and how you can carry an AR with an M4 upper loaded in a vehicle, but not a standard 16″ AR upper.  That’s understandable, because it’s confusing for people who are familiar with Pennsylvania’s gun laws. Any time you look at PA law, you need to be sure which definition of firearm they are using. The standard definition of firearm under Pennsylvania law reads like this:
Any weapon which is designed to or may readily be converted to expel any projectile by the action of an explosive or the frame or receiver of any such weapon.
There are only a few places this is used, mostly for penalty enhancement to existing crimes, and prohibiting NFA firearms if the possessor has no complied with the NFA.
Most of Pennsylvania’s firearms laws are contained in the Uniform Firearms Act. In Pennsylvania, Firearms under the UFA are defined as:
Any pistol or revolver with a barrel length less than 15 inches, any shotgun with a barrel length less than 18 inches or any rifle with a barrel length less than 16 inches, or any pistol, revolver, rifle or shotgun with an overall length of less than 26 inches. The barrel length of a firearm shall be determined by measuring from the muzzle of the barrel to the face of the closed action, bolt or cylinder, whichever is applicable.
So basically, SBRs and SBSs, handguns, most AOWs, and machine pistols and most submachine guns, are considered firearms by the UFA, and are subject to it. That means the extra state police form to transfer one, no private transfers, and you need a license to have one in a vehicle (unless you fall under the exceptions, like going to the range, place of business, etc). If you have a license, you can carry a SBR, SBS, pistol, machine pistol, or submachine gun in a vehicle, loaded, because it’s a UFA firearm.
Long guns aren’t regulated for carry or transport by the state, except for not being allowed to have one loaded in a vehicle. That bit is mentioned in the UFA, and is meant to protect game codes that forbade the practice:
§ 6106.1. Carrying loaded weapons other than firearms.
(a) General rule.–Except as provided in Title 34 (relating to game), no person shall carry a loaded pistol, revolver, shotgun or rifle, other than a firearm as defined in section 6102 (relating to definitions), in any vehicle. The provisions of this section shall not apply to persons excepted from the requirement of a license to carry firearms under section 6106(b)(1), (2), (5) or (6) (relating to firearms not to be carried without a license) nor shall the provisions of this section be construed to permit persons to carry firearms in a vehicle where such conduct is prohibited by section 6106.(b) Penalty.–A person who violates the provisions of this section commits a summary offense.
So basically, you have a loaded rifle in your car, loaded meaning this:
A firearm is loaded if the firing chamber, the nondetachable magazine or, in the case of a revolver, any of the chambers of the cylinder contain ammunition capable of being fired. In the case of a firearm which utilizes a detachable magazine, the term shall mean a magazine suitable for use in said firearm which magazine contains such ammunition and has been inserted in the firearm or is in the same container or, where the container has multiple compartments, the same compartment thereof as the firearm.
I keep an assault weapon bag which has magazine pouches on the outside, and put loaded magazines in the pouches. I would argue that it’s a “separate compartment”, even though it would take 3 second to have a loaded rifle. If I’m wrong, I get a ticket, so who cares, plus, I only really do the truck gun thing when I’m traveling.
You can see that Pennsylvania law is rather odd here.  If you had an M16, you can’t carry it loaded in a vehicle. If you have an M4, and a license to carry, load it up and stick it in the back seat, or stuff it in a trench coat, you’re good to go, and ready to rock and roll. SKS loaded in the back seat? No no. Loaded machine pistol under the seat? No problem!  Weird huh?