What are the weaver/picatinny ring differences? I am planning to upgrade the scope on my G3 so,
I bought a UTG GEN II H&K Staneg Steel Claw Mount
Complete with Picatinny Rail for my GSG-5.

http://www.robertrtg.com/utg_clawmount.html

It took some modifying of both the gun(brand new) and the mount to get a good fit and I am happy with that part but,
why would the put a Picatinny Rail instead of a Weaver on it. The Weaver mounts that I have, the bottom is one machined
piece with a screw that pulls one side in and is sloppy loose on the Picatinny Rail.
Does anyone know about the 2 different standards?
I have not found reasonable priced Picatinny rings yet. Some say they fit both but, I think that would put the scope to one side,
changing the left-right of the bullet as distance changes.

The similarities and differences are summed up pretty well on this web page.

That said, your problem is most likely the rail, not the rings. UTG isn't exactly a "top brand", if you know what I mean. It's possible the rail is undersized, resulting in the loose fit. You could probably check the measurements with a micrometer or a dial caliper, and compare them against the dimensions provided in the link above.

It is undersized! (maybe not, .833, I need to get some of my Weavers out to measure).
Should be .835 with a variance of .005
So, I will just try some of the more adjustable rings at Walmart on the rail.

I did not want grind away at my A.R.M.S. claw lock that is on the G3 and will be used with the next scope.

[This message has been edited by FriendGregory (edited 11-06-2009).]

Weaver rails are often not consistent in their spacing. The Picatinny was designed to be uniform, so that scopes and other accessories could readily interchange from one weapon to another.

And I agree, the rail is likely the problem. UTG isn't a very good brand, and their stuff is wildly inconsistent. It's fine for Airsoft stuff, but I wouldn't bother using it on a real firearm.

If you have trouble finding rings that can clamp firmly to that rail, you could try filing down the mating surfaces on the ring bases (where the two halves that clamp together meet) to let them clamp down tighter.