I am building a lower from 80% to use with a 9mm upper(probably Oly).  Bought some sten mags fromm cdnn(excellent mags by the way).  Got to looking at the lower and had an idea that I thought I would pass on to my fellow enthusiasts.

Take the mag catch slot and cut it closer to the front of the receiver about .75 inch.

using a piece of bar stock shape a knew mag catch approx. 2.2 x .25 x .158 with the correct notchs to look like an extended length mag catch.

drill a hole in one end to fit a .185 inch screw( may have to tap this using the same thread that is on the issue mag catch.  This part could be done a couple of diferent ways I am thinking.) insert said screw the same length and thread as issue mag catch.

using some kind of block made out of either aluminum or steel to give the proper spacing for the sten mag and that can be secured with a set screw(my initial thought is having the set screw to be covered up by the mag catch)

Install it on the upper and use the mag catch slot to align the mags so that a notch can be cut.

Granted this is just an idea and I do not as of yet even have the upper to test the idea with.

tell me what you think.

This is an idea I have toyed with for a while but I just started moving on it yesterday.  Problem is finding/making an extended mag catch.  Long ago I pinned my SOCOM block into my lower, so the first part of the project was to cut it flush with the bottom of the mag well.  Now I have my filler block.

While contemplating the new catch I was pleasantly surprised to find that the lever bar from the SOCOM is the same size as the mag catch! Before I whittle that into a new catch I'm going to take the lower down to the machine shop and have the slot extended.

I'll provide pictures of the project soon.  

By the way, what were you going to use to shape your mag catch?  Do you have access to a mill?

Being a true WECSOGer I was going to use a dremel with a cutoff wheel and grinding stones.[:D]