Hrmm thats a tuffy, but IMO it looks like a .40 slide. I would say, look down from the top (as u are in the pic), and how much daylight u see between the sides of the barrel and the slide. The .40 slide is a tad bit wider and when u put the 9mm barrel in it, u can easily see the difference between them.......If its a .40 slide, all u should need is a .40 barrel. Kinda like everyone who has an actual .40 slide and .40 barrel, all they need is a 9mm barrel. Which ever way u look at it, as long a sits a .40 slide, all u really need is a barrel, lol

You can refer to my video, and get a general idea of what exactly im talking about. You can kinda see everything in the video.

Shield 9mm barrel in Shield .40 frame - YouTube