MIKE GEORGE says: The key to the performance of cartridges is storing them at the right temperature and humidity.
Too hot, like in the airing cupboard, and the pressures go through the roof. Too damp, like in the garage, and they are likely to not go off at all.
Even cold weather will affect them adversely, it is why you never seem to kill those ducks cleanly on bitterly cold days. The exception here are cartridges loaded with double-based powders that can cope with more extreme temperature ranges.
However, because such powders are more expensive few loaders these days use them.
Provided your old Gyttorps have been kept at more or less room temperature, they will be as effective as the day they left the factory.