Dear Andrew,  Thank you for the most helpful tip for cleaning small shells,
as I have  always a lot of problems removing the last trace of animal
matter from the albino version of Calliostoma zizyphrinum f/m lyonsi which
we find locally here in Northern Ireland.  Could I ask what strength of
Hydrogen Peroxide you use; here in the UK we can buy  two strengths 10 vols
(3%)  and 20 vols(6%) in any pharmacy.  The point you have raised regarding
the mixing of bleach and hydrogen peroxide is rather interesting, if you
mix the two you will get effervescence, as nascent oxygen (non-poisonous) 
is released from the hydrogen peroxide immediately, but  nascent oxygen
released so quickly, is unlikely to penetrate the inner regions of such
small shells.  If you were to soak the shells first of all in dilute
bleach, drain them off, so that only the inner surface have a light  
coating of bleach, and then immerse in hydrogen peroxide, the nascent
oxygen  would be released where you need it, as the animal remains will
have absorbed some bleach, and the release of the nascent oxygen may help 
to dislodge it and will most certainly oxidise it.
.As an alternative to bleach, you could experiment with a dilute solution
of ammonia, or washing soda (sodium carbonate) or indeed any alkali.  
Hydrogen peroxide is staple under acid conditions, but in alkaline
conditions, it  becomes unstable 'break down' and releases nascent  oxygen.
 The stronger the alkali the faster the reaction. I hope that you find the
above comments relevant, and not too elementary. I look forward to hearing
the strength of the hydrogen peroxide you use in due course   - Stanley
Francis.