I think it's a longer bet there'll be another run. So your return on investment is going to take longer to realize, and will have that much longer to fight inflation, just like any other investment.
Everyone saw that after Sandy Hook, nothing happened, major anyway, save for states the rest of us already mostly considered to be "screwed" when it comes to RKBA, and the only real "loss" being CO, where we got a major shot across the bow with those recalls in return.
In the meantime several states laws loosened, and IL got shall-issue CCW.
I'd eschew investing in gun parts solely for the idea of flipping them, but instead buy things I think I could want or use, but maybe in quantities where I could afford to flip a few of them should the opportunity arise.