I used an abrasive rubber dremel cone tip like thishttp://www.widgetsupply.com/page/WS/PROD/dremel-polish-rubber-flame/D-GU01
to make the breech just big enough to fit my pellets exactly flush. If you don't have a dremel just use a drill motor. Get several so you can round one or 2 down so they don't go deep into the BBL when you polish it down. You just want to flare the first few centimeters of the port. Keep the pellet you are trying to use handy to check the fit. Once it sits flush you are finished. I kept a cleaning rod in the BBL with a mop end on it while I was polishing to push the pellet back out and keep the dust from going down the BBL.