the one i bought a few years ago is the Bushnell Magnetic Boresighter. it doesn't rely upon different or adjustable spuds that go into the bore. it simply attaches using a magnet. i bought this one because it would work on my Savage rifle in 17 HMR. most 17 caliber spuds are .177 which is the size of most pellet and airgun bores, but the 17 HMR uses a bore of .172, so the supplied spuds for a 17 caliber won't work.
as JonM mentioned and i agree, the first step is the make sure the scope reticle is level first. unless it's level, no boresighter is going to work properly, regardless of it's type or how much it costs.
when i mount a scope, the first step is the stabilize and secure the rifle in some sort of vise or fixture and level the action first. then you can level the scope. alternate tightening the screws and watch your level. use a torque wrench or torque screwdriver to obtain the proper torque on the screws. (you can find these specs usually from the manufacturer of the rings and bases) watch the level as tightening on one side more than the other, will cant the scope from one side or the other.
when i mount the boresighter on my rifle, i measure for the centerline of the scope and the bottom of the scope to the action or barrel and divide this in half and that gives me the approximate measurement of where to place the boresighter on the barrel. i then just line up the crosshairs of the reticle with the crosshairs of the boresighter.
so far i have had really good luck with the Bushnell boresighting tool and so far, it's been no more than about 6-8" off the bulleye at 100 yards.
Chuck Hawks does a review on the same one i have. it's not expensive either. i paid less than $45 for mine a few years ago. i don't know if it will suit your specific needs, but check it out and see.