I don't have a mossberg 500, but the 535. I last year bought the mossberg 535 and I love it, I had the same intentions just like you did Brad. What I found out at the range at 100 yards I had about 8 inch patterns, now I did not have a slug barrel I had a smooth bore barrel with basic slugs. If I were you I would buy the H@R ultra slugger, why because it is the best slugger on the market. If you are looking for accuracy at 200 yards this is the way to go. I don't have one, but from what I have read and heard this is the best. I guy shot this gun at 100 yards he had a .75 inch of a pattern, at 150 yards he had a 4 inch pattern, at 175 yards he had a 5 inch pattern, and my friend has one and he got at 200 yards a 6 inch pattern, and mossberg is going to stop making 500 because 500 can only take a 3 inch shell, but the 535 is identical and can take a 3.5 inch shell. So if I were you I would buy the H@R just like I am going to with scope and all for about 350.00 dollars.
Hoped I helped Buckshooter