a collimator arbor for 20 caliber barrels? My set has a 17 and a 22, and I need to boresight a scope on a 204 Ruger.

Wrap a little tape around the 17. It should get you on paper.
One nice thing about the Leupold bore sighter is it does not stick inside the bbl but is held on by a magnet.

I can highly recommend the Laserlyte although I know you are only looking for an arbor. I took one to Zim with me last year and it was easy to check the zero at 100yds right at dusk. You don't have to worry about height over bore or any other calculations just put it in the muzzle tighten (it self centers well) turn on and check crosswires at any distance you can see it. Really slick and takes up hadly any space and weighs next to nothing. Comes with tips to fit almost any range except .17.

Try Sweany.