...or owned any guns at all?
The presence of guns in a house, owned by the adults, is a kind of power play. Legal or not, taking the guns of that person sends a message. About who is in control.
After reading the last few posts, I'm thinking that stealing from a neighbor is highly, highly unlikely. Would be a family member OR some internet transaction.
If you recall, the store from which BS bought his armband and which sold certain Nazi-themed items closed. Maybe BS and KM snooped around under-the-table retail and managed to find some underground source for weapons- pay cash, no records, something totally illegal.