This is what I use. With the one I have, I can see the laser spot on the 100 yd backer from the bench. Put your crosshairs on the target and the laser beam defines your line of bore. Use a flag to locate the beam.
MidTen Bore Sight kit Green Laser Sight BoreSighter
Best 33 bucks you'll have spent. The unit is chineese, so yours might not work as well as mine. Send it back and try another one if you have trouble. The plastic bore plugs furnished work good enough but I plan to make one from nylon to fit my bore. As it is, the spot is about 6" off at 100yd.