Unknown WW1 Shell Casing

[Airforce1]

Friend has this casing from WW1 but can't Identify it. I don't have the dimensions handy so all I have to go on is the pictures alone. Apologize in advance for the crappy cell phone pictures. Anyone know what this is?

Danke

[AKKO]

I thinks it's a 77mm.

[Airforce1]

I thinks it's a 77mm.

Thanks! Any idea what kind of gun this would have been used in?