I just had to copy this from GrrlScientist 'cause my favorite1 daughter loves M&M's and physics.
Here's the Science paper that calculates the packing fraction of M&M's.
Donev, A., Cisse, I., Sachs, D., Variano, E.A., Stillinger, F.H., Connelly, R., Torquato, S., Chaikin, P.M. (2004) Improving the density of jammed disordered packings using ellipsoids. Science 303:990-993. [PubMed] [doi: 10.1126/science.1093010]
1. and only