For a minute Tuppence thought she was going to spring upon her, which would have placed the girl in an unpleasant dilemma, since she meant to draw the line at actually letting off the revolver.
Tuppence did not have any thoughts about shooting a gun at anyone. 
mnli hypothesis: Tuppence did not have any thoughts about shooting a gun at anyone.  premise: For a minute Tuppence thought she was going to spring upon her, which would have placed the girl in an unpleasant dilemma, since she meant to draw the line at actually letting off the revolver.