the number that shows the elevation angle goes from -30 to + 30, but the gun itself only goes from ~-10 to ~+20, so once you have reached these limits but move the mouse further, the number will still rise, giving you an incorrect angle once you aim back at a ground target.
the m4 scorcher has a kinda similar issue (http://feedback.arma3.com/view.php?id=16653) which is still not fixed in 1.12
place tank in editor
switch to gunner seat
move cannon up and down to the angle limitations
get results like 8° for a target directly ahead
Confirmed. It reads the neutral position as that in which empty vehicles spawn, with the gun elevated. Pointing the gun horizontally thus turns into a negative value.
The indicator also reads beyond the limits of the gun as OP said.
The reported bug still happens in the latest dev build (1.53.132856) with M2A1 and M2A4 tanks.
In addition the Tigris elevation indicator becomes absolutely unhelpful in normal operation.
Repro steps for Tigris: