Whether a polymer (rubber, plactic etc.) shrinks / expands is dependant on the composition of the compound. For example think about tires and the performance characteristics avaiable, these characteristics are all obtained through the the use of various polymers (up to 6 / more in a tire each with a specific "job to do"), carbon black, here again there are many varities based on carbon chain lenght etc., zinc oxide, if white is desired, oils - again many available, UV inhibitors, anti-oxidants and so on. So if a buttplate may shrink, expand, disintegrate, harden, soften, crack etc. is all dependant on the initial composition, it may turn out good or bad over time. Even where the polymer is kept will have an effect, eg. in sunlight will effect its type of aging (see above), usually only time (sometimes years) can tell the outcome. FWIW --- John Can.
