Not quite right Woodsy. It needs two horns, because the rounds feed alternatively from either side of the Mag. The underside 'Projections' have no bearing as a positive shoulder. As the case sits firmly on it's base in a recess. It cannot move up or down in the recess. So therefore, as a positive shoulder as you call it. That is superfluous!...
Now, go back to your desk & try again young man! see Me after school!........LOL

good try though, well done buddy.