ive seen mossberg 500 scope bases at d- icks sporting goods for 12$. 40$ sounds too high. if you want long range accuracy for that 500 just get the rifled slug barrel with the cantilever(scope base) already on the barrel for aroun 150$-its great if you want to use a scope and be able to change the barrels out quickly. if you dont want the extra barrel then i say put a adjustable sight that clamps to the vent rib. if you dont have the vent rib then i would go with a scope. its not legal in my state to hunt with slugs without an adjustable site.shooting slugs in a smooth bore can be accurate but i wouldnt expect more than 75 yards which is more than probably most people shoot and the ammo is way cheaper than the sabots. you just have to try different brands. federal has a slug that uses a ball behing the slug. supposedly makes the bullet fly great. im gonna try them soon but good old remington and winchester lead slugs have been good to 50 yards. you can also buy a smooth bore slug barrel cheap here http://www.havlinsales.com/