I have some 38/357 loads that use very little powder.  I use a fired brass and cut discs from the foam trays that my beef steaks come in from the grocery store.  I make my cutter by opening up the primer hole in a used unsized fired brass and using a headed nail to push the disc out after it is cut.  I powder all of my primered cases, confirm all have same powder charge, add one foam disc, push disc against powder charge with a standard #2 pencil (eraser end) and then seat the bullet.
In my loads, this disc, cut and sized by the fired brass, is just oversized enough to compress a little in the sized case and it keeps the powder against the primer end.  I get low SD and have yet to get a squib load.
Some folks have said that using a wad in a case can cause rings in a chamber.  I have not found this to be true with my loads.  In preparing to do my own 45/70 loads, I have purchased fiber wads that are similar to original wads that were made for black powder loads.  I do not see where this use of wads can cause the rings, but I am not a "ring" expert.
The foam that I use appears to disintegrates and I have yet to find any trace of it in barrel or brass. 
Good luck with your project.
Steve 
