I recently bought a Mossberg 500 from a shop in which it had sat collecting dust for some time - never used but it lacked a manual.
Point is that i intended to use it for wildfowling but haven't been able to confirm to what pressure its barrel is proofed to and whether i can use steel or hevishot in it......any guidance would be appreciated.

Does the barrel have screw in chokes? If it does be sure the choke tube says it works for steel. If your 500 is an older fixed choke model and it has a full choke, you will want to have the choke reworked depending on the size of steel shot you are planning to shoot. A full choke will pattern okay with 4-71/2 shot. As the shot size moves up the choke should be more "open" and depending on whether you are shooting over decoys or pass shooting. I like to tailor the choke to the size of shot and the type of shooting, But if you want one choke for all around shooting with steel it would be best to have a modified (.020 smaller than the bore).

cheers mate. It's the screw-in type. It was bought new quite recently.

Most of the newer chokes are marked if they are intended for steel shot, if there is no markings it is wise not to use the full choke tube with anything but lead.