I'm sure this manga is a fun read - but I won't be reading it. Getting past the first chapter when the author knows nearly nothing about firearms is tough. First of all, you don't hold a handgun (or really any gun) with your trigger finger stuck all the way through the guard like that, something I admit is so ridiculous I've never seen it before. You can't stop time fast enough to stop a bullet fired from point-blank range, but you could argue he did it earlier and it was just artistic license. In one scene, there's an empty case on the ground as it should be, but then a few pages later, the whole cartridges are being fired from the gun apparently.
The joke was supposed to be the premise, not the technical details of the content. If you can get past that, go ahead and read it, it's probably good for some laughs.