I have used stripper clips for my AR for many years. As noted above, the secret is pressing down near the primer end of the cartridge, not the middle. The adapter I use is the same type given to our military. I think the vernacular for it is a "spoon." Loading up a batch of stripper clips before heading to the range certainly reduces reload time, but on the down side, you will burn up a lot more ammo when it's this easy.
Crusader Rabbit