@klaypigeon, is there any way you can post your Capx?
Essentially, when you spawn a sprite with bullet behavior, it will automatically set at the "default" speed the bullet behavior is set at. So you need to make sure the speed is set for Every spawned bullet if it varies from the default.
Example: So right after you have Gun spawn bullet, set speed of bullet behavior on bullet sprite to variable that changes on a different event.
May not be what your looking for, so a capx might help :)