when using the particlegun module to shoot kaons with uniformly distributed momentum into the detector, geant produces a good amount of secondary pions whose total momentum has sharp peaks. the attached graph shows the distribution of the total momentum (in gev) stored in mcparticle for pions whose mother was a kaon. since the kaons are uniformly distributed in (0, 1.1gev), i'd expect a relatively flat spectrum.

for generation i used the july release + externals v00-01-00, with no special options given to fullsim.