If you want to make an enemy with a machine gun, use the FSM, see the FAQ on the wiki.
If you want to create a special explosion, also use FSM. particle command would be helpful.
The sound of shells can also be made using the FSM.
By increasing the margin of the bullet projectile image, the firing point can be moved.
If you want the bullets to fire from the lower right corner of the screen, increase the transparent area in the upper left corner of the bullet image.