67mm is too long, but none of us can predict the pressure increase as the thin plastic case enters the forcing cone. From Bell's study possibly as much as 1000 psi
Details here about 1/3 way down
https://docs.google.com/document/d/1ZIo0y746UsSRZIgRuuxwAbZjSBHitO_EanvwLYc-kGA/edit

Please let us know the chamber length. The index card technique works just fine
http://www.lcsmith.org/faq/chamlgth.html