I use the bushnell laser bore sighting tool. I comes with a laser and a bunch of different arbors for different bores. It was only ~$25 and works really well. The laser is a little dim, but not bad. If you are using it at high-noon in direct sunlight, you may have to put your target in a box.
I chose that instead of the cartridge style laser because I can use it for many different calibers and cartridges. I shoot a lot of .30cal stuff and with the cartridge laser, I would have to buy a different piece for each: .308, 30-06, 30-30, 300WinMag, etc, etc.