Here's the scenario,i want a trigger volume to fire it's output as soon as the player is completely inside the volume,how can that be done?

Well, a simple solution would be to reduce the size of your volume to take in consideration the collision cylinder of your character. This way you fake the delay with the distance.

If you want something more precise, try 2 trigger volumes (One in of an other and try a double check tot rigger your event).

Or you can try to code this specific trigger volume.

The 2 trigger volume and a gate solution can work,interesting trick...thank you froyok.