when i had my AR, i owned one, i think it was 120 round though, im not sure.... anyways, it wasnt the beta type, it was the chinese typer, and i found the last 20 rounds wouldnt load properly. Not sure if i didnt wind it enough, but its either a 120 rounder that doesnt feed the last 20, or a flawless 100 rounder :P

PS, its the one pictured here:

http://www.biggerhammer.net/ar15/magazines/chinese.html