First, make sure your shotgun and barrel are approved for slugs. Second, make sure you are using the proper type of slug for your barrel (is it rifled or smooth bore). As to the drop of the slug, go to the website of the manufacturer to see the chart for your particular round. If you only have a 18 1/2" barrel, the drop may be more than the charts show. See if they list what length of barrel was used. Most slug barrels are 21" to 24".