They are all a waste of money. With a bolt rifle, set the rifle on a rest and line up the crosshairs with a target at 50 yards. Look through the bore and note how much you are off, and in which direction. Adjust your scope and fire 1 shot. I zeroed 3 rifles and scopes Tuesday. These are the 1st shots from each rifle at 50 yards using this method.
Trust me, none of the boresight tools are this accurate, and the ones with lasers are the least accurate.
I adjust my sights and fire the 2nd shot at 100 yards. Adjust again and shot #3 will be perfectly zeroed. I don't waste time or ammo with 3 shot groups until after I use 3 shots to zero.
If you cannot see through the barrel, just use a large sheet of poster board for a target, they are 28 cents each at Walmart. It is impossible to miss. Even if the 1st shot is a foot or more off all you have to do is measure the distance and count the number of clicks before making your 2nd shot.
I've never needed more than 4 shots to get zeroed and can do it with 3 with a bolt rifle. I can't see on of these gadgets ever saving me a single round of ammo.