I'm honestly not sure. I think it's automatically illegal to possess a gun by PA law if you are a prohibited person by federal law or something like that but I am not a lawyer. I know that if you fail a background check the prosecution is done locally under PA law so it would make sense.

I would assume then that being caught with weed and a firearm in PA could be prosecuted even with a MMJ card, since you're a prohibited person by federal law. Supposedly just having a valid MMJ card (from any state) is evidence that someone is a user, so likewise I think being caught with just an MMJ card and a firearm (e.g. in a traffic stop or a visit to your house for something unrelated) could be prosecuted.

I don't know how likely that would be, but If I had an MMJ card, I would absolutely keep it 100% separate from anything firearm-related all the time. That's not a risk I would take but I'd guess most people would be OK for years.

What I would truly worry about is if the PSP were given access to the database again.

They already have access to the database(s) of people with a License to Carry Firearms . If they had access to the database(s) of MMJ card holders, it would be a pretty simple task to compare the names on one to the names on the other and see who lands on both. Anyone with an MMJ card older than their LCTF has automatically committed a felony and they have their signature on the proof. They can be prosecuted with just a little bit of paperwork. Everyone else could have their LCTF immediately revoked at least since they're known to be prohibited.

I'm not sure what they are allowed to do with the record of handgun transfers, you'd have to ask a lawyer. It wouldn't be hard to match people up, but I don't know whether they could prosecute you if you had an MMJ card when you did a transfer, but it's also a felony since you lied on the form and that would technically be proof. I think they could more easily prosecute anyone going forward. It probably wouldn't be difficult to link the MMJ database(s) to the PICS check and fail you when you try to buy a gun. If they didn't do that they could still easily cross-reference afterward and prosecute you just as easily.

I don't recommend owning firearms and holding an MMJ card at the same time, but if someone decided to do it, I'd suggest not buying any handguns or new long guns and absolutely not applying for (or renewing!) a License to Carry Firearms as long as you hold it.