The tikka box mag is generally pretty hard to seat all the way in. Yours is probably a little too tight and not getting fully latched. Try loosening the action screws a little bit (don't shoot it with loose screws), and if the box mag snaps in all the way then you can put a shim under the bottom metal, you shouldn't need one more than .050 of an inch thick. Make sure you put a shim under each of the action screws.
Scheme well, scheme often.
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