Hey, thanks for the comments!
actually there is a little angle on the back, some stay trapped in the bottom,
the main difficult when shooting at this little plates is that you need to shoot them pretty centered to make them flipped, if shoot on sides or too high, seams to not have enough energy to flip them. This worked too with my little boys crosman 1911 springer airsoft pistol that are rated at+/- 300fps... haven't try it yet with my KJW M4!... I think I will put some ductseal in the back.