There was someone whom once said "I'd rather be lucky than great." In some endeavors, Lady Luck just may save us from ourselves.
It seems as if we do things as an expedient that actually work. Maybe not the best (shop for caps??), nor the safest (shop for caps??)
but none the less, get it done.
I have to admit, I wasn't around playing with Cap Guns in the "good old - bad old" days before we had access to really good after-market nipples. However, I personally would have found a way to alter my existing nipples to give a better cap fit, before I resorted to squeezing an impact sensitive explosive device between my delicate, pain sensitive and highly valued (by me) fingers. Let alone some loon putting the squeeze on with his TEETH.
Physics 101 - - an explosion will follow the path of least resistance. The convoluted route, around the lower skirt of the cap, up the side of the nipple, a u-turn into the flame channel then down into the powder seems somewhat ......... unlikely. There are easier routes for the
blow-back and cap flash from an adjacent chamber to escape. I am, however, conversant with ....... Murphy. Never completely discount
anything Murphy can effect. A chain fire from pinched caps
? Meh ....... Murphy ......... Ump
However, these are MY fingers. We do have a Stirling example on our very forms (pictures included) of why one should NOT seat caps with one's thumbs. So ..... Why would I pinch one with MY fingers!!!! Silly me.