Using a SOCOM block--
Any of ya'll figured out how to circumvent the whole "Sten mag over-insertion issue?"
Example: When the bolt is locked back and you insert a mag prior to dropping the bolt release. It is far too easy to push the mag in too far, thereby causing the bolt to slam on the back side of the mag (and not loading a round immediately).
Bran new sten mags have half dimples that stick out on both sides of the mag. They have to be pressed down a little in order to fit into the AR mag well, but if you press them down to far, the Sten will go right through like your describing.
Yours must be pressed completely flat, pry them up a little bit so they stick out more.