Yes, bb's do just drop into the barrel. I've carved a tiny lip on the barrel that prevents them from coming out unless you really really shake it hard or set off the mine.
Easy reloading of powder. I've been thinking about this and here's my idea. Ever put a straw in a drink, cover one end with your finger and contain some of your drink in the straw? Well, same idea, except I guess the ends would have to be taped, and then just fill the whole thing with powder before hand. Then when it's time, just let it transfer and slide out into the other barrel. Also, though the powder is better, it could be used in emergencies (or if it was raining and would get wet anyway) without the powder. You still get the sound and a smaller pft. effect of CO2. I'll work on getting out a video showing how easy it is to reload.
Cratedriver, you asked what I used for a spike. It's on the parts list a couple posts back, cushioned nail-on glide or something is the technical name. The beauty of the design is that I don't have to secure it, it's just put in and is secured, which is great because the only maintenance this mine requires is the occasional (for optimal results that is, you can use it 20 times in a row no problem) sharpening of this spike, and since it's not fixed to anything, you can take it right out.
"To invent, you need a good imagination and a pile of junk." - Edison