If it's 9mm Browning Long, 9x21 brass works well.
Those Astra 400s are nice handling pistols. Usually very accurate as well.
Back in the mid seventies, I was visiting a friend on an Austrian military base. They had a couple of hundred of them, that hadn't seen any use since WWII.
They all looked brand new and were all in 9x19 chambering.
Supposedly, they are plenty strong enough to handle the hot submachine gun loadings the Axis used to issue and as such were preferred side arms for those that carried pistols into combat by many of the Axis participants.
The fit and finish on them is phenomenal.
If my memory serves, Germany and Austria both ordered several thousand of them from Spain during WWII. The facilities in Spain, were not capable of producing and delivering such large quantities quickly.
The pistols I saw, just outside of Vienna, were part of an order that was made during WWII but not delivered until after the war was over. They had been paid for and under international agreement, had to be delivered.
I've always wondered what happened to all of those lovely pistols
They are definitely top quality and relatively underappreciated.
Very few people realize how much action Astra pistols saw during WWII.
I wonder how many the Russians have hidden away????