I used a pair of the Z-rings on a one piece Leupold base for my slug gun and I love those rubber inserts. If the point of impact is off, you can buy the adjustment rubber inserts that allow you to adjust point of impact without using the knobs on the scope.
If you provide me with the make and model of the gun, I can probably help you out with picking the correct rings and base.
You can also go to
www.burrisoptics.com and look at the fit chart. I think this is the website for the fit chart:
http://www.burrisoptics.com/mountfit.html
You just need to find a dovetail base that fits your gun and 30mm dovetail z rings.
Disclaimer - I am not a professional (i.e., I don't get paid to install scopes), but I have spent several hours looking at rings and bases to avoid having to buy Weaver type mounts.