I did some research for you and found this on richard durso's site.............
Metal Halide bulbs have an internal nipple which usually must always face upward. If you fail to do this the bulb may not light, or it could flicker. This seems to be very important when using electronic ballast to power your metal halide lights. (Actually, the nipple itself does nothing, its just a reference. However, the glass bulb has an internal vessel that has a slope. This slope is upward from left to right if the base of the bulb was on your left side.)