It is damn neat!

Comments under the original design on DA suggest that the problem is the sealing between the cylinder and the barrel. It was not explained in enough detail but I imagine that with such long barrel, the leak of gases would be bigger problem than in the case of regular short-barreled revolvers. If the barrel is short, the bullet will leave it swiftly. But with a long one, the pressure has time to build up some more, overcome the sealing and cause bigger leak.

But here, we can assume some bit of advanced near-sci-fi material engineering keeping things under control.