Converting 20 round Sten mags back to 32 rounds

Discussion in 'Firearms' started by Tully Mars, Apr 14, 2017.


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  1. Tully Mars

    Tully Mars Metal weldin' monkey

    Sten magazines can and are often adapted to fit several different weapons, and until recently were really cheap. The recent rise in popularity of the AR-9 (AR-15 chambered in 9mm Luger) and other 9mm carbine type weapons are probably the biggest reason for the price jump from 5 dollars a mag to 25 dollars per and sometimes even more.
    Lately there has been a lot of the 20 rounders come on the market. These are 32 round magazines that have been blocked to only hold 20 rounds, and they can be had for around 5 dollars apiece at this time. I recently purchased 100 for my Stens and the Colt style AR-9 that I recently built up and I thought some here might like to know how to convert these mags back into what they were originally.

    First thing you will need to do is to take the mag apart and give it a good cleaning. I put mine in a parts washer and scrubbed all parts with steel wool or a scotch brite pad. The magazine will come apart easily by pushing in the button on the base plate and then sliding the base plate off. CAREFUL!! The spring is under pressure! SLOWLY slide the base plate off with the thumb of the hand that is holding the magazine while using the other hand to cover and catch the spring as the base plate is removed.

    20170409_141117. 20170409_141139. 20170409_141958.
    Once the spring is removed, the follower may or may not slide out. Some of mine have, and some did not. No biggie at this point.
    The way these mags were blocked was by brazing 4 small brass rods inside the magazine body, 2 on each side, blocking the follower from traveling down past a certain point inside of the mag body. In the above pic, near the bottom you can see where the rods protrude through the mag body and are bent over or toenailed. This is the next step. Using pliers or small side cutters (what I used) gently pry the brass rod up so it will pass through the holes. The brass used is very soft and may break off while you are prying the end up. No worries if it does.
    20170409_142209. 20170409_142213. 20170409_142310.
    At this point you should be able to gently push or tap the rods back into the magazine body. If needed you can reach in with a pair of need nosed pliers and give them a gentle tug.

    20170409_142147_001. 20170409_142157.
    In the above pics you can see the two points where the brass rods were brazed in place. There will be two of these places near the top of the magazine body on the left AND right sides. Sometimes they are a bit hard to see which is why we clean the mag first.
    Using an 1/8 inch drill bit centered on these spots drill them out. It doesn't take much, and is a lot easier if you have a drill press and vice, but can be done with a hand drill as well. Should look like this when you are finished.
    20170409_142521_001. 20170409_142625. 20170409_142827.
    Above is what the rods look like after they are removed. Other than taking a small file to remove any burrs from the drilling that's it! Once you clean all the parts (esp the mag body inside and out) the magazine is ready to bead blast and recoat with the coating of your choice, or simply lube and reassemble. They are now back to a 32 round capacity as God and Her Majesty originally intended;)

    On a side note, these are destined for the bead blaster then some are to be coated using DuraCoat's DuraBlue while the remainder will be coated to match my AR-9. If there is an interest I will post follow up steps for those processes as well as what it takes to put a Sten kit together legally.

    One more thing.
    For those of you that have a torch and can braze, after cleaning these little rods work pretty well as brazing rod with a little flux!
     
    Last edited: Apr 14, 2017
    Gator 45/70, arleigh and techsar like this.
  2. techsar

    techsar Monkey+++

    Lucky to have found mags like that...the conversions I've seen had steel rods running up each corner and appeared to have been spot welded in place.
    Might be time to reexamine the marketplace :)
     
    Tully Mars likes this.
  3. Tully Mars

    Tully Mars Metal weldin' monkey

    I would need to see one, but I'd bet you could drill out the spot welds.
     
  4. techsar

    techsar Monkey+++

    I passed on them and got "virgin" examples...

    BTW, interested in your progress & kit build...
     
    Tully Mars likes this.
  5. Ura-Ki

    Ura-Ki Grudge Monkey

    Mine takes H-K .45 mags, but the idea is close to the same!
     
    Tully Mars likes this.
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