I think the problem with steel is that is conducts heat really well, and could cook off primer or powder much earlier than wood, for instance.
The Manufacture and Storage of Explosives Regulations 2005 (SI Number 2005/1082) came into force on 26 April 2005.
It has very specific guidelines as to what's ok, and what's not.
I was very surprised as to some of the new requirements.
I'm confused as to magazines being made of metal too, but I think metal is ok in commercial applications because of additional requirements, (like it has to be against a cement wall, and there can be nothing within 6 feet of it), wood is much preferred in residential settings.
I know people who use old fridges for this, I don't see a problem with that personally, but they wouldn't be in compliance with the 2005 update if they had over 20 pounds of powder in their fridge.
The regulations at SAAMI are for under 20 pounds, once you get over 20 pounds, there are additional requirements.
It's really easy to accidentally get over 20 pounds and it's just easier to overbuild when you start, so you won't have to start all over if you want to buy in bulk.
I'm not just a gun.
I'm YOUR gun.