Some more interesting insights from Sam:
Two reasons for this stuffing practice.
Liquid core (Mercury or oil) keeps spinning after contact imparting a retro effect. Other materials either add weight or damp the shock wave through the boule softening the reaction(Rubber bands, metal swarfe, springs).
Old players can be very sneaky; The German team at the last European Championships in the “Veterans” category in MONACO, were disqualified for boule stuffing in Oct 2016. In the Lyonnaise game this is perfectly legal, in fact they are manufactured with a variety of fillings but as they start at 90mm diameter you can’t get away with it in Petanque. Boules when made must be within + or- 5g of stamped weight.
THERE IS IN FACT A WEAR AND TEAR RULE:
Weight loss due to wear and use in play may not be greater than 15 grams below the marked weight.