Good Question, I've wondered the same thing. A while back I borrowed an older Laser Boresighter from a friend. It had a light unit that you slide into the chamber, as best as possible with a wire hanging out the back, and a smaller orifice tube in the muzzle. The larger light was reduced by the muzzle tube and you'd get a nice red dot out at a target. Good in theory, not great in practice. From what I see in the catalogs, newer versions must be much better.
However, This year I've sighted in three rifles and never really thought a bore sighter would help. Equipment wise you need to have a good gun stand or bench rest or set of sand bags, and a good spotting scope. I probably sighted in all three rifles with less than 15 shots, and the last one was only a 22LR so I played around with it a bit.
However, if your sighting in "ALOT" of rifles, a good boresighter of any kind is probably money well spent.