I've have a single shot 12 gauge that was gifted to me just sitting around, and have always wondered about those caliber conversions. Reading up on them the consensus is they work, in that the round will discharge in the general direction you're pointing, but that's about it. How could I pass up a chance to try that? Picked up one of the 9mm rifled versions off ebay for $30-ish delivered. It's a nice piece of machined steel with good looking rifling, drops in without drama. With the breach closed though, you can hear it rattle a bit. Not a good sign for accuracy. The only non-steel case 9mm I have that isn't defensive ammo is some speer 147gr subsonics. I fired a hand full of them trying to hit the 100 yard gong without the benefit of any sort of sights on the shotgun. Based on the impacts on the berm 6' right then 3' left then 4' high, then 3' low.... I'm going to say it's a solid 90 MOA gun in this configuration. Perfect, if the target is big enough. Never once missed the berm. Spent casings fell right out. Kinda surprised about that, figured they might stick which is why I didn't use steel cased 9mm. Report was handgun loud, and recoil was nothing. There is a guy on youtube who's showing off a red dot mount for these single shot guns, I'm half tempted to try that and see what accuracy is at 25 yards. That, and/or do the mod I've seen some of these come with in adding an o-ring to tighten up the fit. Worth it? Probably not.... except it doubles as a high quality 9mm case gauge. So in that respect, maybe it is.