I've never taken a Mossberg O/U apart ....but I routinely take my Browning O/U's apart / at least semi-annually for general maintenance.
Assuming you do not have an adj comb / you will probably find 2 screws in the recoil pad - hidden beneath a couple of slits in the rubber of the pad / that are probably phillips head screws. To get the recoil pad off / you have to find those screws and remove them.
Then you will find a hole in the stock ( maybe 12" deep ) / and where the receiver mates into the stock - there will be some kind of a stock bolt. Some gun mfg;s use bolts with allen heads on them / but many of them have a standard screw headed slot. You will have to use a light to take a look ... then probably use a long screwdriver to loosen the stock bolt.
Note: if you have an adj comb / you usually have to remove the comb first / so mark how you have it set with a felt pen or something so you can get it back on in exactly the same position ( height, cast, etc ..)...
A note of caution / stock bolts sometimes have a little bit of "lock tite" on them - so be firm / but don't go all "gorilla torque" on this bolt or start smacking it with a hammer / or you might crack the stock. You need a screw driver that fully fits the slot / and firm pressure to loosen it. I hold the receiver in my hands with a terry cloth towel / to make sure it stays tight. Be persistent / and I think you'll be able to get it loose.
After you fully loosen the stock bolt - the receiver pulls out of the stock ( straight forward ) .... and now you can see the springs, firing pins etc ....and I've never seen a gun where anything is loose / or parts that just fall out ...but go slow and look at it carefully.
Don't mess around with the springs, firing pins unless you really know what you're doing. Clean it out / use a Q tip / and spray some light oil ( a preservative oil ) like Rig oil on the springs, etc ...and emphasis on "light" ... you never want to saturate the wood with oil. If you have rust, or pins are in bad shape ... you need to find a good gunsmith to help you at this point. If an O/U has been shot in rainy weather / or stored where it didn't have a chance to dry out ...it can be a rusty mess in there.
Whenever I've shot in real wet weather / I pull the stock off the receiver when I get home / let everything dry out real well -- spray some oil in it / re-assemble the gun and put it in the safe ... / and once every 6 months ( on a rainy day when I'm looking for something to do - I pull the stocks off all of my O/U's and just check them all.
Once you get the hang of this ...it only takes about 20 min per gun ...its not a big deal. Note: when you tighten the stock bolt back up ...don't get too aggressive / snug it up ...don't over torque it ! Hearing a loud
"snap" would not be a good thing ....like in "wood cracking" ...but this is routine maintenance that every gun owner ought to be able to handle.
