Although I think it would make the game less playable and intuitive you could always try something like this:
create a sprite that will represent your mouse-pointer, give it the bullet-behaviour and (turn of set angle) set the speed to the speed you want..
On start of layout set the mouse pointer style to none
set the angle of motion to angle(sprite.x,sprite.y,mouse.x,mouse.y)
set the speed to your desired speed..
if distance(sprite.x,sprite.y,mouse.x,mouse.y) < a certain amount
set the postion of the sprite to mouse.x,mouse.y
set the bullet speed to 0