I have found an answer to dry firing my Marlin XT-22 without worrying about damaging my firing pin or chamber edge. I was using empty .22 LR brass but they wern't easy to chamber. I found plastic screw anchors that match the rim width and rim depth measurements of a .22 LR. They are from the local ACE Hardware store. They are a uni size 4-6-8 x 7/8". They are made by Hillman and the Hillman Stock Number is H#370326. They are easy to chamber, will eject, and should be good for 10 to 12 strikes around the rim. A box of 100 was $5 and is good for more the a thousand dry fires.