As I've written a number of times before, the highest and best use for a "boresighter" is to help in getting a scope reasonably aligned with the bore during mounting. Part of the "problem" is that ammo doesn't always shoot exactly where the bore points.
We've all seen how various rifles will put different loads into different places, even after one particular load is sighted-in, however that's defined. So there's no way for a collimator will insure every kind of ammo lands near the center of the reticle. But by trying the collimator on several sighted-in rifles, we can comes up with a reasonable average.
Given all that, I have had good luck with the Wheeler laser collimator (green beam), given that the point of light tends to be lower than the center of the reticle at, say, 15-20 feet, about as much space as available indoors. Outside it's pretty darn accurate at around 30 yards, partly because the magnet's circular area is over an inch wide, so you can slide it up and down on the muzzle to approximate the scope's height above the center of the bore. It's especially helpful on calibers where bore-spuds aren't small or large enough, whether on a .17 Hornet or 12-gauge slug gun.
