Shrapnel arrer is not always an instant cast ability.

[ToXoS]

I made some tests, and Shrapnel arrer is not always an instant cast ability, despite its description.

If used just after another shooting ability with a casting time, it's an instant cast, even on the run.
If used just after another shooting ability with instant cast, it takes 1 second for Shrapnel arrer to cast.
If used first, shrapnel arrer takes 1 second to shoot, even if there is no cast bar.
If another shooting ability is used right after it, it doesn't make shrapnel arrer instant cast (still 1 sec cast).

For unknown reasons, sometimes even on the run after another shooting ability with a casting time, shrapnel arrer is a 1 second cast.

Is it a known thing? Sometimes it's an instant cast, sometimes not. It seems random to me.