In a saucepan, combine Karo syrup, sugar, water, and vinegar.

Cook over medium heat, stirring constantly until the mixture boils.

Continue cooking, stirring almost constantly, until the mixture reaches the hard ball stage (260°F) or a small amount forms a hard ball when tested in very cold water.

Remove from heat and quickly add butter, blending well.

Slowly pour the syrup mixture over the popped corn in a large bowl, mixing thoroughly to coat all the popcorn.

Form the coated popcorn into balls using as little pressure as possible.

If desired, butter your hands to prevent sticking while forming the balls.

Let cool slightly before serving.