I have seen two different types of the end of barrel boresighters one of them has different caliber rods to insert into the barrel and then the rod attaches onto the bottom of the bore sighter, the other type of bore sighter attaches to the end of the barrel by a magnet.
I have a LaserLyte. Works pretty well, it's a universal so it'll fit just about any caliber barrel. It uses 4 plastic inserts to fit the barrel which are infinitely adjustable (uses a screw to tighten or losen the insert which causes it to expand or contract). Basically to use this you pick an insert that a bit smaller than the barrel then when you screw it onto the bore sighter it expands a little, then you fine adjust it to have just enough friction inside the barrel to keep the bore sighter level. Seems to work.