I have an ithica flues sxs 12ga shotgun from approx. the 1940s or 50s id suppose. Maybe earlier. It was my great grandfathers. It appears in good condition with nice bores and such but it has a strange habit. If you crack it open carefully just barely letting the breach open so you can extract shells, theres no issue. If you break it open with any force or let it swing all the way open, you will hear a click and it will now refuse to shut. Happens if the firing pins are released or not. Makes no difference. Pressing the breech release (i don't know proper terminology on this style of shotgun) or triggers or any combination of the 3 means nothing either. I have to remove the front grip by popping it off. Then it will easily swing back into place. I don't notice any unusual looking markings on the pieces. If anyone has any ideas on where to start that would be awesome. I'm typically pretty handy with working on my own weapons, this is just more of a family heirloom but i also dont have big $ to spend on taking it to a gunsmith. 

Forgot to add. It doesn't lock completely open. After the click it will still swivel to a point. Basically it will let it close until the shell extractors come in line with the firing pins. And its not the extracts being jammed and hitting the pins. I've made sure of this.

Ithaca Flues were made from 1908 to 1926. The Ithaca NID was introduced in 1925 to replace the Flues and continued in production until 1948. You can check the serial numbers here: http://ithacagunrepair.com/IGSN/IGSN.html

 

Tough to find a schematic for this one. And the Flues and NID differ internally a bit. Like most doubles, the disassembly starts with the trigger guard and then the trigger plate. If it's a Flues, you can start reading about it here:

 

http://www.google.com/patents/US1070441?printsec=drawing#v=onepage&q&f=false