I've had one of these for 4 years http://www.brownells.com/.aspx/pid=9...ER_OPTIC_SIGHT

The magnet is pretty strong, it comes with a couple different beads, and a couple different widths.

I cracked it this year when the muzzle hit an overhead branch, but it's not really the product's fault - it was bitter cold, something like 11 deg. A little epoxy fixed it, but for $20 i could have just bought a new one.