Initial development tests used printer paper rolls to achieve the proper thickness. The resultant product deformed significantly to be used, and wasn't replicable by the average person. Subsequent attempts using whittled bamboo sticks with paper rolls produced good results, but subpar capability to reproduce the results.
The goal then became to provide a consistent means of producing a 1:6 scale shotgun shell that YOU, the reader, could feasibly do in the comfort of your own desk, given the same tools I have. You probably have better things to do than make your own bullets, but it's always good to know you can save a bit of money for other things. Like more guns.
Using an existing Dragon shotgun shell as a model, I measured the dimensions and found that I'd need a 7.5mm long 3mm diameter cylindrical rod, and a 3.175mm diameter end cap about 0.5mm thick with a minor depression in the center. Those dimensions are mighty convenient... A 1/8" hole punch on two Magic: the Gathering cards glued together gets me the end cap dimensions. As for the 3mm diameter rods, I happen to know the perfect source!
GUNDAM runners. I ended up keeping my runners from my Airmaster, Aegis and Blitz Gundam kits. I got a few more from the Obitsu Multi Purpose Clear Stands I bought. They both have diameters of 3mm for the runners. Excellent. And there's also runners that come in red! There's enough runner from one kit (or in this case, one Obitsu stand) to just cut down the smooth sections and ignore the sprues and gates. You only need 7.5mm of material and there's enough material to make about 40 rounds or more. I chose to smooth out the runners to form relatively smooth clear plastic rods, sand them lightly, then trim them down using a Dremel. A saw or knife works fine, but Dremel's for the impatient builder. Can't be bothered waiting 2 weeks, nor 5 minutes of sawing. Got to have my results in 3 seconds.