I'm not really well versed in that area, but my understanding is that to do a prop gun requires you show special exempt status.. IE., A licensed production company or similar...See that's what I was trying to get at. There are people that use unmilled 80% lowers to build prop guns as they can't have a trigger on them to pass security prop check and because they are normally cheaper than building one.
This is where it creates the gray area, are the prop builds illegal to have as well now? Having an unfinished receiver is included in the bill and undetectable receivers i.e. plastic lowers. So are owners of those props now criminals?
Also if you only have just an unmilled lower and say you're going to use it to build a prop, can they still charge you with having it since building and having props isn't illegal.
The average Joe/Jane would not be exempted.. or rather... be able to claim the "prop gun" exemption. I'm sure there are some guidelines/requirements involved that they must comply with, but I don't know that for a fact.. or what they might be.