all the manufactures (British, german French aso) tells you to use a wad one caliber bigger when you use brass case.
for example if you have 12 gauge all brass case you must use a 10 Gauge wad.
Indeed the tickness of the wall of a LIGHT all brass case is smaller than for a paper case.
Light brass cases are for example a French light SFM case, or a Perfect Kynoch or an Eley light case.
(The 10-12 from Eley is nothing else than a light case and they did put in the hstp 10 to show you can use it as if it was 10 Gauge and therefore use a 10 wad. It is not a special caliber despite what say the people selling them for high price ! Eley did that for marketing reasons because they had no success with their light cases. A chamberless gun was even designed for this case, and they advertised in such a way. This is stupid because ANY all brass case from any manufacturer can chamber this gun!)
For an HEAVY brass case (SFM , Eley Kynoch or German ones) you should use same wad as the diameter (12 wad for a 12 Gauge).
You have to be carefull with this fact (a wad one size bigger than the one hstp on the LIGHT metallic case ) when you deal with big bores (2, 3 4, 6, gauges).
Because the 2, 3, 4 gauges not a 2, 3 or 4 gauges !!
(4 as definded in England, French or US is actually a 6 gauge. Only in Germany the 4 gauge is a true 4 gauge)