Doing this all over again, my first post crashed. I went off on a tangent searching for one of these guns, after digging for answers, and I found one!
First off - Owners Manual if you don't have one downloaded yet...
http://pdf.textfiles.com/manuals/FIREARMS/jchiggins_30.pdf
Hard Copy if you don't like free...
http://rstmanuals.ecrater.com/p/12483811/jc-higgins-model-30-factory-instruction
A few suggestions here on mount...
http://www.gunandgame.com/forums/general-rifle/117739-jc-higgins-model-30-groved-reiever-mount.html
Thread here...
http://www.thehighroad.org/archive/index.php/t-258415.html
Contains this post...
Quote:I have a J.C. Higgins Mod. 31. As for the scope rings, I bought a set of 22 cal. rings from Sportsman Guide and filed them down so they would clamp down tight. Worked great. I know it's not keeping it original but, a 3-9X40 scope is really sweet on it.
I also found another post suggesting that it is an "11mm groove".
Not sure which is correct but, all threads elude to the same odd sized scope groove, on all JC Higgins 22 rifles.
At one point it was suggested that Mosberg was the manufacturer, but I found at least a dozen different references stating they were made by High Standard.
Everyone says they are great shooters, but they're getting some age on them and a few folks are encountering minor problems.
Personally, I think the guy quoted above with the regular 1" 22 mounts and filing them to fit is probably the easiest solution, I'm not finding any reference to available parts. But, I'd say the Dremel tool with the drum sander attachment, duct taped to a match box car, with the mount set up on a stack of playing cards so you can add minute increments as necessary, while rolling the dremel back and forth, would be almost machine shop like. (o;?
Nawww... just use a good sharp file and keep your strokes smooth and level. You should be able to measure the difference in that mount and a standard 22 mount and know how much to take off.
Hopefully some of that helps because, seriously... I did inquire about one. It's a cool gun! I gotta git me one of them!!