[Mr. 16 gauge]

Forget all the crap about reloading rice, bird seed, ect.....the best way to make a popper load is to use cardboard wads. Fill the case with the powder charge, place the required number of cardboard wads on top, and crimp as you would a normal load. It may take a little experimentation to get the correct number of wads needed, depending on their thickness, the type of hull, powder charge, ect.
I have also loaded 20 gauge blanks without a reloader by cutting down the plastic on the shell & loading the cardboard wads by hand and then finishing off the last wad on the top with a coat of clear nail polish, and they work just as well.
When firing these loads, Rocky is right.....they can be lethal at close range, even with the cardboard wads, but the cardboard wads will shred as they come out of the barrel. The shredded debris looks like a bunch of feathers from a hit bird, so it helps to add realism to the scenario when I am dog training (which is why I load poppers in the first place). Also, the cardboard wads are biodegradable.
Cardboard wads are avaialble from both Precision Reloading and Ballistic Products, and a lot of muzzleloading shops carry them as well.
I will check my notes later and see what the powder charge is....I got the recipe from a reloading book by Don Zutz.