would it be reasonable to use some dry wall tape to shim a butt grip which is one size to big for the blank ?

Try this instead - make a wrap of D thread, leaving perhaps 1.8th inch between the threads, all the way along where the grip should reside. Coat with epoxy and then screw the grip in place.

If D won't do it, try E. If it's much larger than that, forget shimming and get a grip with a smaller ID.

.............

thanks again TOM

Hank,
Just take masking tape and build up the blank as needed for a nice fit. Between each wrap of masking tape as you go down the blank, leave about a 1/16th - 1/8th inch space. That way, when you glue up the blank, the masking tape will keep the blank centered on the blank. Then, the epoxy on both the masking tape as well as the grooves between the layers of tape to lock the grip in place. Dry wall tape is fine, but masking tape works just fine if you leave space between the rows of masking tape on the blank, to give you a nice epoxy lock from the grip to the blank.

You could also use a foam arbor, but if you are only talking a very small difference, it is very difficult to ream out an arbor for this thin space without ripping up the arbor.

Take care
Roger