I usually swap stocks or trigger groups for lock-up. But, you can shim by adding thin strips of wood along the bottom of the stock where the trigger group fits into the stock. It's the back edges of the area. You can add wood and then sand down till pretty tight or experiment with some shims that are not affixed.
The Walt Kuleck Garand Complete Assembly Guide is a nice book and the Duff Owners Guide. If you really want to get serious get a copy of Kuhnhausen's shop manual.Information
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