Great, but has issues.
I'm quite familiar with your Sift Heads series, so I suppose the whole 'monochrome environment' some people were criticizing was not that big of an impact. I find the black silhouettes against the gray background to be easy enough to spot but hard enough to force a second or third scan of the environment. Therefore I chalk that one up to screen contrast being a potential problem, but that's on the player's side.
I believe that issue with your 'Cool Slow Motion Element' is the fact that the targets still move in that time frame. While this is probably intended with the bird targets, this shouldn't be applicable to the sniper targets who are placed closer but somehow move faster than the birds during the travel time. This usually results in scenarios where I make a shot at a concealed sniper but within the travel time he moves and it becomes a miss. Note that the movement speed is the same as without the slow motion effect. As others have mentioned, it also gets a tad repetitive and having zoomed out after the shot adds to the frustration as well. I recommend limiting this feature to headshots or make it optional altogether.
I've noticed that the rifle is rather inaccurate when not using the scope. This is fine, but you need to reflect this in the crosshair design. You'll notice that many FPS games make their crosshairs larger to signify a wider cone of accuracy/fire. On the flip side, a tight and static crosshair with a dot implies dead-on accuracy.
You also need to ensure that your shots can't penetrate cover, unless that was part of the design. If that was the case, adding bullet holes and the like will probably make the players less confused as to their shot placements.
I have no problem with the snipers taking cover or the opaque blood in face part of the game, a challenge is a challenge after all.
You've obviously put a lot of work into this, and I believe it to be your best game yet. If you do get the time and motivation, the I suggest you consider the following:
1) Make the sniper target movements more natural looking. There are times where the elite sniper just pops up in the background, it's way too obvious.
2) Eliminate movement for sniper targets during the bullet travel time.
3) Design a crosshair that correctly reflects the (unzoomed) accuracy of the weapon.
4) Environment/object scarring.
5) Reload function is not needed, you can use this to limit the number of bullets available as an extra challenge.
That's that really, keep up the good work.
