Actually...and you're probably going to roll your eyes at me...The I robot hero gun in fact has a visible seam, as it was a clamshell design that went over a revolver.
That being said though, the one in the pic is one of the solid stunt castings.
Which was made 'off register' so all castings of that gun have that seam and annoying offset. Which adds to the visible seam line due to the lighting angle.
M