Here's my favorite range toy, which I used as my training wheels for going through the NFA process for the first time right before the ATF dropped Rule 41F on us.
Walther HK 416 clone in .22LR. Can shoot the little bugger all day long for less than a nickel per round. With cheap Amazon red-dot and laser, it's fun for all sorts of plinking games.
Of course, nowadays hardly anyone bothers to do SBR builds, thanks to the ATF's relaxed ruling on stabilizng braces. I'm still glad to have set up my gun trust, because you never know when rulings might get reversed, and suppressors are still NFA items.