You did nothing wrong. The feed lips are too tight on your factory mag.
When it does this, the feed lips are too tight.
Too tight...
Just right....
Just use a Dremel or fle to open it up a bit. Make sure your follower has enough tension as well. It's adjusted by twisting the spring.
And before anybody asks -- I don't have any. The damn ram is still stuck. It hasn't worked a day since I've had it....period.