Aside form the legalities of things, open bolt guns also tend to have some safety issues you dont have with closed bolt guns, and people who are familiar with them.
First and foremost, the open bolt guns look to be in a more or less "safe" condition to those familiar with closed bolt guns, when in fact, they are ready to go. With the closed bolt guns, the open action usually indicates the gun is safe or empty or not in a condition to fire. With the open bolt guns, when the bolt is closed, they are actually "safe". (Before the safety nuts jump in, when I say "safe", I mean more or less "safe", not totally safe. You still need to follow normal safety and handling protocols)
Another issue is loading and unloading the gun, or making it "safe" for carry.
With the closed bolt guns, you simply insert the mag, work the charging handle, and put the safety on.
To unload, drop the mag, open the action and lock the bolt back.
With the open bolt, you have choices. (This is where you really need to pay attention to what you have in your hands and know just how it works before you start putting live rounds in it. There are a lot of different guns out there and many work differently, even those of a similar or same type.)
One, with the bolt closed, you just insert the mag. If the gun has a safety lever or switch (some do, some dont), you can set it, which usually locks the bolt. If it doesnt lock the bolt, or you choose not to lock the bolt, there are other issues. Some of these type guns are not drop safe, some have bolt locking devices, that help, but are no guarantee. If you drop the gun, its very possible that the bolt can move rearward enough to strip a round and fire the gun.
Two, you can insert the mag with the bolt closed, then retract the bolt to a cocked position. If the gun has a safety, you can set it, but it can still be tricky depending on what you have. Many of the basic SMG's, like the MP40, STEN, etc, have a safety "notch" in the frame or tube, which takes the spring load off the trigger and puts it on the charging handle. Thats really the safety when the gun is cocked and not ready for immediate use (well, sort of).
Three, with many open bolt guns, you really need to be very careful when you pull the bolt back. Most have no means to keep the bolt from going forward if your hand should slip, and if it does, the gun is likely to fire when you do. As long as the bolt has cleared the rear of the round on top of the mag, it will strip it off the mag as it goes forward, and at that point, usually has enough momentum to fire the round when it chambers. With any gun you have to make sure you have the weapon pointed in a safe direction when you charge it, but its even more critical with an open bolt. Its also critical you have good trigger discipline. With an open bolt gun, it will run until its empty or you let off the trigger. If your finger is on the trigger when the bolt slips, it probably wont be just one round going out.
Unloading is another issue. You must remove the mag before you lower the bolt. If you dont, the gun will fire. You'd be amazed at how many people dont get this. You'd also be amazed at how many still have their finger on the trigger when they do. While the results may sound comical and like something out of the Three Stooges, its usually a pretty scary event for all involved.
Once you get the gun unloaded, you lower the bolt with the mag out, and then reinsert it, or just leave it empty all together.
Another thing many open bolt guns also have is, pretty light triggers. At least many of the ones I've shot have. Lighter than you'd likely expect anyway. While it does take a little getting used to with the bolt moving before the shot, you can actually get pretty decent accuracy out of them once your used to them.
But if I know TFLers, I'll get lots of great information without being made to feel like an idiot. I hope.