Originally Posted by old guy
I've heard the terms side lock and box lock. But don't have any real understanding of them. This is the first double barrel I have ever had anything to do with.

On a sidelock the lock mechanism is mounted on the plate as in the L.C. Smith, Baker, Crescent or Meriden --

[Linked Image from i.imgur.com]

In a boxlock, the lock mechanism, hammer, sear, mainspring and sear spring are mounted in the frame as in the New Ithaca Double --

[Linked Image from i.imgur.com]