I doubt they would ever or could ever prosecute because I have the items capable of committing a crime.
Would they prosecute? Maybe not.
Could they prosecute? Absolutely.
Your examples are bad ones. A better example would be:
"There's a bag of cocaine under the passenger seat of my friend's car I'm riding in. Can I be charged with possession even though it's not actually on my person?" The answer, of course, is yes, and it happens all the time.
The concept of "constructive possession" means that you don't have actual possession of contraband, but have the ability to exercise dominion or control over contraband. When applied to NFA firearms, if two parts that can be easily combined to make an unregistered NFA firearm are in close proximity, and they have no other obvious legitimate purposes, then you can be charged with constructive possession.
It has happened before.
However, along the lines of the "would they prosecute" question, you have to realize that the people who are generally prosecuted under theories like this were already on the ATF's radar for something else, and this is just one extra charge they're tagging on to really ruin someone's day. There's not an epidemic of ATF agents hanging out at public ranges and busting people who have AR15 pistols and stocks nearby.
But that doesn't mean it can't happen. It just means it's unlikely.
I have the items capable of committing a crime
Just to give you one more example from a state where we have a serious methamphetamine epidemic:
You can be convicted of manufacturing methamphetamine if you have the intent to manufacture methamphetamine and possess two or more items of equipment for the manufacture of methamphetamine. A whole bunch of household items are used to make meth. Do you have coffee filters and gas-line water remover? Do you have lithium batteries and plastic tubing? The only thing stopping a prosecutor from charging you with making meth if you have any two ingredients in your house is his belief about whether you intended to make meth.
So obviously the vast majority of us don't use meth, have never used meth, and don't intend to make it. The cops aren't gonna bust down our doors and arrest us. But if you're a meth-head and they catch you with this stuff, you may be in trouble, even if you truly weren't gonna make any meth.
Same thing with NFA constructive possession. If you just happen to have the parts necessary to make an unregistered NFA firearm because you own an AR15 pistol, an AR15 rifle and multiple stocks for the rifle... you're probably not going to get busted. But if you're also trafficking cocaine across state lines, or you're also building illegal machine guns, or you're also doing anything else the feds might be watching you for... then you might end up with a constructive possession charge.
It
could happen to a lot of people. It most likely won't. But it's still good to know the law and be safe.
Aaron