If you have a loading jag, put a clean patch on the end of the jag. Now push the patch with the ramrod and jag, down to the breech. The patch will catch most of the cap discharge. Now with all that in place, pop a cap or two. This is actually a good idea because you not only feel the trigger pull but also test the rifles ability to properly fire a cap.

When you are all done with your testing and playing, put a clean patch on the jag and swab the bore. Then apply some gun oil on a new patch and swab the barrel with that. Primers are said to be non corrosive if you believe that.