TFB Armorer's Bench: Converting 20rd STEN Mags Back to 32rds

Welcome everyone to the TFB Armorer's Bench! This series focuses on home armorer and gunsmith activities to help you improve your knowledge, enrich your skills, and maintain your firearms. Today, I'm tackling something that caught me off guard when I started building my STEN Mk2 - converting those Indian-modified 20rd magazines back to their original 32rd capacity. When I bought my parts kit and started shopping for magazines, I kept running into these 20rd mags and wondered what the heck was going on. Turns out there's an interesting history behind this conversion, and more importantly, it's completely reversible with basic tools.

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TFB Armorer's Bench: Converting 20rd STEN Mags Back to 32rds

Here, we at TFB hope to inform, entertain, and even inspire any would-be gunsmith or armorer out there. Ideally, with the information I provide and with the help of our sponsors, you can have some useful knowledge pertaining to the conservation and improvement of firearms technology while at the same time sharing experiences and teaching each other new tips and tricks along the way in the comments. Digging deep into what it is to be an armorer or gunsmith has significance but what is important is what those people do to show they’ve earned that title. I am happy to share my experiences and knowledge and hope it is informative!

Make your personal safety a priority:

1. Practice proper gun safety. Always make sure before the firearm hits your bench that it is unloaded and safe to be handled.
2. Wear the proper safety equipment. The main one would be safety glasses (decent ones) since parts are often under spring tension and you may work with high RPM tools. Other honorable mentions would be latex gloves or a respirator when working with potentially harmful solvents and oils. Also hearing protection when working with loud machinery or test-firing firearms.
3. Modifications, alterations, and customizations will void your firearm’s warranty 9.5 times out of 10. Please take that into consideration before attempting any at-home gunsmithing.
4. If you are unsure about proper safety practices, disassembly procedures, or warranty standards, stop, put down the tools, and consult a competent gunsmith.

The STEN Gun and Its Magazine Problem

The STEN submachine gun represents one of Britain's most pragmatic wartime designs. Facing desperate shortages after Dunkirk and needing something cheap to produce quickly, designers Reginald Shepherd and Harold Turpin created a gun that could be built for about $11. Nearly 4 million STENs rolled off British production lines during WWII, making it the second most-produced submachine gun of the war after the Soviet PPSh-41.

The Indian Central Railway Solution

After the war, the Indian military inherited thousands of STENs and quickly discovered what British troops had been complaining about: the magazines were a weak point (one of many. It's a crude “tube gun” after all). Rather than just living with unreliable feeding, the Indian Central Railway Workshop took action.

Their solution was brilliantly simple: convert the troublesome double-stack magazines to single-stack by inserting brass rods inside the magazine body.

You'll sometimes find these converted magazines marked with "I.C.R.1" stamps, indicating the First Workshop of the Indian Central Railway that performed the conversion work.

Methods I've Seen

Before I get into how I approached this, let me cover the different techniques I've come across. There are really two parts to this conversion: dealing with the brass rod tails at the bottom of the magazine, and removing the rod heads that are ground flush with the magazine body at the top.

For the bottom tails, I've seen people use flathead screwdrivers to pry them up until they're sticking straight out. Others go the brute force route with pliers, either prying them up or just clipping them off entirely. I even saw one guy using a sharp chisel to shear the tails off by running it down the magazine body, though I'd recommend using a beater chisel for this, not your good woodworking tools.

My Method and Tools

Here's what worked for me, keeping things simple and practical:

Tool List:

• Punch (for floor plate removal)
• Small hammer
• Flathead screwdriver
• Heavy-duty pliers
• Hand drill with 9/64" bit
• Safety glasses (non-negotiable)

Step-by-Step Process:

First, I removed the floor plate. These plates are bent over the magazine sides and can be stubborn. Insert your punch and press the floor plate against the internal base plate that's being pushed by the magazine spring. While holding this pressure, give the back of the floor plate a few light taps with your hammer toward the front of the magazine. This creates a gap where you can insert your flathead screwdriver and pry the plate forward. Watch out - that magazine spring is under tension and will launch out if you're not ready for it.

Here's the thing that made me understand why the Indians did their conversion in the first place: you can hear and feel the cartridges dancing around at the bottom of a full magazine. There's definitely more play in the double-stack configuration, which explains why the single-stack conversion improved reliability even if it sacrificed capacity.

I haven't had range time with my STEN build yet to compare reliability between original and converted magazines, but the theory makes sense. The Indian military wasn't making this modification for fun - they were solving a real operational problem. I just like the idea of having things as original as possible. 20rd mags are cheap enough still if I wanted more.

Final Thoughts: Converting 20rd STEN Mags Back to 32rds

Converting these magazines back to original capacity is straightforward work that doesn't require special tools or skills. The hardest part is finding the STEN magazines these days, but when you do come across bundles of the 20-round Indian conversions, now you know they're not some oddball variant.

Whether you keep them as 20-rounders for that authentic post-war reliability or convert them back to their original capacity is up to you. Both have their place, but for a semi-auto build where you're not doing mag dumps, having the extra rounds is probably worth it.