I learned on the original M-16. My Instructors taught me to place my cheekbone on the stock. I fired expert. I never fired competitive rifle. I Captained the 5th Division Combat Pistol team. In 1976.
The tube on the Collapsible stock is about 2 inches lower than the top of the original M-16 stock I shot for some 22 years. None of my instructors, all of whom were Vietnam Veterans with 6 bronze stars, 4 Silver Stars, two Distinguished Marksman awards between them said any thing about putting my nose on the charging handle.