I was buying one or two HP mags from Midway whenever I could get them for about a dollar a round. I have several with different followers and some have smooth sides and some don't. Floorplates are different too. However, all work. I haven't tried any of the 10 rounders, don't see the point. The South African 17 rounders from CDNN work too although the springs are stiff. Have you found any 13/13 + rounders that don't work?