I built my Shockwave before they were production models, 590 barrels do fit on a 500,but I found the gun severely lacking
I just put a pistol brace on it a couple days ago and now have a fully functioning gun with an actual purpose other than a range fun gun.
Over/Unders, Doubles, Pumps, Semis, Home Defense.
I picked up a Tac14 DM for grins and giggles and couldn't be happier. I went with 870 since I'm an armorer and like the platform. The FCG is definitely not as nice as the old school metal type, and could use some smoothing out with some sandpaper. High brass isn't that fun to shoot, low brass is much more palatable, and a pistol brace would definitely be good. Fore-end could use a strap.