HepMcParticleLink_p3 uses unsigned int rather than unsigned short to hold the eventIndex. This is a step towards preventing the aliasing of higher event numbers seen with HepMcParticleLink_p2. This merge request does not change the transient HepMcParticleLink class, so it won't fix the problem on its own. The downside is that it will increase the size of events on disk. The size of this effect will need to be determined before the change is merged.