While you're waiting for a better solution from a more experienced user, here's two solutions I can think of:
1. Define a new family called 'SolidObject', and add all your solids to this family. Test for a collision with 'SolidObject'. The down side to this is that every time you create a new solid you need to add it to the 'SolidObject' family.
2. This is more hack-ish, but add a Platform Movement behavior to your bullet and set all the attributes to 0, and the 'Default Controls' to no. Now Platform Movement doesn't really do much except give you a new condition to test for. In your events test for 'Is by wall' for left and for right. Use an OR to test for both sides, and destroy it in the action. The benefit of this is that you don't have to worry about classifying all your solids.