That's gonna loose you velocity since the wadding will no longer check the gases in the barrel before it leaves the barrel. I'll stick with my 20" Mossberg 590. Oh, if want to intimidate them I'll fix the bayonet.
Will this device actually cause the shot to penetrate less compared to a normal barrel? Seems like the velocity would be the same but the pattern would be more dispersed? No, no, NO! They aren't ILLEGAL... they are more restricted!
Yeah, I think that I'll keep shooting my 590 stoked with 00 Hornady Tap...for a tight shot...why would I want to spray an entire room?...and where did the rule of knowing your target and beyond go? If you spray a room, someone that doesn't need to be hit will be hit.
Ballistically, this thing is a joke. Modern powders still require 10-14" of barrel to burn completely. Thus by venting around the wad before then, you are losing a great deal of velocity. With the mouth that wide, the wad will be tumbling when it comes out. I would think a better design would be some porting around 12 to 14" down the barrel (with perhaps even a little necking resembling choke) that might shave a little wad, braking it and taking it out of the picture. Then open up the end like a tuba, with perhaps even some imperfections in the barrel to get the shot started bouncing around and hitting each other.