Is this security chain to be used inside a secure premises, inside a securely locked cabinet of sorts to secure your airguns from unwanted removal ? Long gone when your airguns were ok under the bed or in the sock draw.
If the above security measures applies, a simply cheap but effective way could be curtain wire,with eyelets big enough for a quality padlock to fit, quality rather than size, passed through a larger eyelet stuck to the side of cabinet, Bostick' Sticks like S**t is brilliant, and that will secure your airguns from unauthorized removal. Unless the unwanted remover comes tooled up with bolt (wire cutters)cutters in which it won't make a lot of difference as your prized collection will be badly damaged while having unauthorized removal.