With guide bush work, if you’re using a bush of small diameter, make sure that your depth of cut isn’t so deep that the collet comes down on the guide bush. If it does, you’ll damage both the bush and the collet nut.
When I plunged my DeWalt DW621 collet onto the top of an 18mm guide bush the bush was damaged and pushed out of shape and the collet nut showed a silver ring where it tried to ‘drill’ through the bush. I replaced both.
Given a choice of guide bush diameter (e.g. when you’re making your own templates) a bush with a diameter big enough to let your collet through is a useful safety measure.