Well the model 500 Mossberg takes both the 2 3/" and 3" shells. It all depends on the barrel you have. The barrel should say what it will shoot/hold. Also, what choke system is on it? If it's the cylinder choke you should be fine. If it's the select a choke or other, you just have to have it at the right setting I think. So you don't blow the choke out or ruin it.
My Mossberg model 500 is marked 600At and had a 28" vent ribbed barrel with the select a choke. Could have improved, modified or full just by turning the choke. Still have the barrel but I put an 18 1/2" security barrel with cylinder choke and bead front sight. Did some other but since I didn't hunt with it anymore figured I'd convert it to a home defense tactical shotgun. I still have everything so I could/can always put it back to original. lol
Do you have a picture of the barrel or a way to post a picture of it to help us better help you.
In fact, go here
>>> page7b <<< and save to file and also print the manual for it if you don't have one.
Scroll down to M and find Mossberg model 500,505,535,590 and 835.