Thanks for the idea. And by the way, that is an .222 cal rifle shell that I have cut in half.
Yeah, this probably not a good idea, seeing as gunpowder can be ignited by friction. Unless you took 5 minutes slowly sawing, you risked the shell firing.
Or was the shell already fired, and only the casing?
As for the bbb, bbb's painted black just seem bland to me. I would buy some purple paint (darker than the original) and do a bit of detail work. But hey, it's your gun, if you like it, use it. If not, go out and buy another. That's the beauty of nerf.