How can I modify a grooved scope receiver to a weaver mount.

Are you looking to mount something that can only go on a weaver mount? If that's the case you could have it drilled and tapped. If you want to use a 1" scope on a rimfire base, Tasco sells bases with integral rings that fit rimfire grooves but have 1" rings.

 

And they work VERY well indeed!

 

Some years back, I bought a couple pairs of 3/8 groove-to-Weaver adapter blocks at the local gunshow. These have worked very well, allowing me to use scopes on a couple Norinco .22 rifles (Chinese-copies of the BRNO action) that had bolt-handle-clearance issues with standard scopes. They mount to the gun's grooves with set screws. I haven't seen them in awhile, so I don't know if they are still made/sold? I'm thinking Tasco made them at that time.

 

curious, what kind of rifle we talkin here

 

There are many adapters that make it real easy. Here is an example:

MidwayUSA - B-Square 3/8" Aluminum Dovetail to Weaver-Style Adapter Matte