My first prototype I took a piece of pvc the same size as my barrel, cut it in the middle lengthwise. I then Lined the cut pvc with wax paper and taped the pvc back into its original postion. I then took expanding foam and filled the pvc with it until it was 3/4 full. Then I let it dry out. After it...

The best way I have tried this is taking a 5" piece of a water noodle and cut it in the middle length wise. Hollow out the inside to place your bb's. Then put the two pieces back together and ram it down your barrel. It gives a nice spread.