What's the law and how do you stop them going rusty?

Q: What are the regulations 
for the secure storage 
of shotgun cartridges?

A: Provided that you are an ordinary certificate holder and not a dealer who is storing cartridges in bulk, there is no regulation such 
as that which exists for rifle ammunition. You are not obliged 
to lock cartridges away or store them in a security cabinet. However, it is common sense to ensure that your ammunition is stored safely, especially if there are children in the household, and to do so would be good practice. At the very least, ensure that your cartridges are kept out of sight in a place that is not subject to damp or excesses of temperature.
Make sure that cartridges are 
not visible to any casual visitor or through a window where they will 
show that 
it is likely 
a shotgun is stored in the property.

one cartridge in shotgun

Q: I want to keep shotgun cartridges in the loft, which never drops below freezing but does get a little damp at times. I am not talking about keeping the ammo there longterm, just for a few months. What do you suggest?

A: I am quite conversant with the problems involved in storing large numbers of shotgun cartridges in the average house.

I find the best plan is to pack cartridges in airtight containers. I have some big plastic boxes with lids that seal tightly, although I suppose you could use well sealed polythene bags. The important thing is to pack the cartridges in a dry, warm atmosphere before you seal them up, so that you don’t trap moisture in with them.

Don’t spray cartridge heads with rust repellents like WD40. Evidence from America suggests that it can penetrate the primers and make them unreliable.

Steel shot cartridges review

Steel shot cartridges: Can steel shot be used for pigeon shooting? Lewis Potter tests a variety from his cartridge bag.

There seems to be some disagreement about the shelf-life of plastic-cased cartridges, so all I can say is that I have some which are around 15 years old, and they still shoot perfectly.