Assassinate will often shoot a regular bullet and unscope.

[Oninaig]

Very often with Assassinate, you will instead fire a normal bullet, which effectively means you're doing nothing.

Here are some video examples. Keep in mind in each video where I scope in and aim at the target I am actively clicking M1 to fire the sniper shot, but it either doesn't fire or simply fires a normal shot and unscopes.