I've got more gun parts than I probably know about; and more guns than I can remember. I'm sure that I have got several foregrips for my AC556 (only one mounted at a time
) I bet if I tried, I could get one of those foregrips to fit one of my pistols that has a rail on the frame. Does that make me technically in "constructive possession" of an unregistered NFA item? Somehow, I tend to think no one is willing to try to make that case to a jury just because you can figure out how to make something illegal out of all of the junk you have lying around. In other words, unless you are trying to make illegal things with your extra foregrips that are meant for weapons they can be used on, I wouldn't worry too much about constructive possession.