A very simple solution that doesn't use any detectors at all:http://www.fileshack.us/get_file.php?id ... hitbox.cap
The "bullet" (in this case, the big blue square) has two animations, one that is it's "display" animation which gets drawn to the screen, and the other which is it's "hitbox" animation that does the collision. The hitbox animation is smaller than the display animation (32x32).
Before doing any collision checks with the player, set the animation of the bullet to "collision." After checking collision events, set it back to "display" so that Construct will draw the correct sprite. Construct only draws the screen after all the events have run, so you will never see the bullet showing it's collision box animation.
In this example you can see that even though the player (brown box) can walk onto the "bullet," he doesn't get knocked away unless he touches the center of it.
This should do exactly what you need it to do, and in fact would be a good way to do custom detectors in general without using separate objects.