So I had several mosins in my inventory stashed up for the mosin quests, but I haven't been doing said quests and they were taking up space. I wanted to sell them, but not for the pitiful 15k Mechanic gives for them. That's when I realized that the only reason they lost FIR status was that I modded them. After I took the scope and muzzle brake off they regained their FIR status and I was able to sell them on the flea.
Figured I'd share. Here's screenshots of the before and after.