I used a weaver #62 (from info on this forum), which fit perfectly on my factory tapped receiver. I got it from Brownell's which was $7.95 (with mounting screws), about a year ago. After agonizing for months, I finally opted for the Ultra Dot Matchdot. I like the fact that I can vary the dot size and brightness for various distances and disciplines. Its just a matter of what kind of shooting you do.