Re: Is it safe to use Steel zinc plated bb's as ball bearings?
03rivers is correct, the ball bearing material shouldn't be a problem as far as safety is concerned.
As for the plastic bearings, most people use AirSoft pellets. I believe you can get them in either 6mm (about 1/4 inch) or 8mm (5/16 inch) depending on what you need. I use a 6mm diameter plastic bead that I had left over from some long forgotten craft project in my KM. The bead has no hole in it BTW. It still needs to be kept clean in order to function properly but it makes purging much easier than with the standard steel bearing.