need abit more information than that.
Are we talking old style, newer freak thread, and if newer style for what marker?
Edited:
Newer freak style -
http://www.smartparts.com/Products/AllAmerican.aspx
.689 for Cocker and Shocker, others .693
Older style memory is going abit fuzzy here, the backs were different colours to designate the bore. Bore sizes for the light and dark grey backs might be incorrect.
Dark grey back - .693
Light grey back - .687
Black back - .689