They made an steel extension for the magazine to add weight to 1100 and 870's. Might find one on EBay or perhaps a old Skeet shooter has one laying around. I have added weight by replacing the plug with a steel rod or just placed a lead plug under the cap that holds the spring in under the screw on cap. I added a lot of weight by drilling a hole in the cap and added a bow weight to the cap. Moved the weight past my hand and smoothed out the swing.
First thing is to figure how much you want to add be taping weight under the barrel. 2-4-6 ounces can make a big difference. Then figure which works easier steel or lead if extra is needed. Inside or out is just a personal choice as far as I am concerned.
