Ricky: Smoothbores come in different diameters, and every gun needs to be measured with calipers to get an idea of its exact bore diameter, in thousandths of an inch. It makes a difference, not only in choosing loading components, but in cleaning equipment.
Measure first. Then choose a jag size for the barrel. If the bore is smaller than normal, you can chuck it in a drill, and use a file on the soft brass to reduce its diameter so that it works for your barrel.
For the most part, the same jab sold to clean a .62 caliber rifle can be used to clean a 20 gauge smoothbore. BUT-- there is always a 'but'--- You will find different dimensions on jags sold commercially, depending on who makes them. I have three cleaning /loading jags in .50 caliber in my range box, and they were all different diameters when I bought them. I didn't have my micrometer with me when I bought the first one, but I did when I bought the next two. Even the clerk in the booth where I bought the last two was surprised that they were different dimensions, as they came from the same sub-contractor.