not sure about 20ga . 12 GA is about .100 thickness, little less for thin wall tubes. I have cut three or four in 12 ga , win chokes ,all seem to work fine. I don't have access to a 20 ga tap so never tried it. The important thing is to use a guide bushing. there is s pilot shaft on the front of the tap, you need to measure ID of barrel and turn a brass bushing that is .001 undersize. Fit this on front of tap and it keeps the tap straight.
