Conversion of Two Piece Colt Style Block to Accept Sten Magazines
Disclaimer: This is only a reference. The mechanical ability of each individual varies. Any people that
follow this document take the responsibility upon themselves. The end result is totally dependent on the
ability of the individual conducting the work. If you do not feel comfortable doing any of the modifications,
it is suggested that a trained individual perform the work.
This reference is the “how to” in converting a Colt style magazine block to accept Sten magazines. The
block that was modified was obtained from Brownells (part number 505-015-009 $120.00). It is the JRW
two-piece magazine block for the Colt style 9mm “AR” system. The JRW magazine block is held in by
“set screws”. The reference material below will be for a JRW block. If you have another two-piece block,
you will have to examine your block and make note of the differences.
Tools:
Calipers (0.000”)
Drill
- Drill bit (just under ¼”)
Dremel
- w/ cut off saw blade (i.e. mini chop saw blades…get the ones that are reinforced)
- Carbide cutter
Roll pin punches or an appropriate sized punch
Hammer
Hack saw (sharp one!)
Flat metal file
Round metal file (large and small)
Scratch Awl or small screwdriver
Observations/Comments
When looking at the unaltered block, one will notice that the pieces have flanges on the front and back.
The purpose of the flanges is to position the Colt type magazine in the 223 magazine well correctly. The
223 magazine well opening is approximately 0.923” wide by 2.379” long. (These measurements were
taken off of an ASA single piece block, as the block fits snugly in the lower.) A Sten magazine is
approximately 0.887” to 0.905” wide and approximately 1.500”to 1.530" front to back. As you can see, the
width of the Sten allows it to fit inside the 223 magazine opening. To contrast, a Colt type 9mm magazine
is about 0.851" wide and it is about 1.331" long. The Sten magazine is too wide and too “long” to fit in the
opening. Hence, the need for modification of the block.
Back Block Modifications:
The modifications to the back block are the simplest to perform. All that is needed is to remove the flanges
that position the 9mm Colt magazine and to remove the “hold open tab”. The JRW block has a tab that
allows the 9mm Colt magazine to activate the stock 223-bolt hold open. This hold open device can be
removed one of two ways. Cut the tab off (quick way with the cut off blades & Dremel) so that it does not
extend pas the block or drive the pins out and remove the entire piece completely. The flanges on both
sides are removed. This can be done w/ the hacksaw. Cut the flanges off flush with the block. Using a
hacksaw allows better control then using a cut off blade and the Dremel. A file can be used to clean up the
„cut‟ area. As seen on the picture, you can see where the flanges were cut and filed as with where the hold
open tab was positioned.
Front Block Modifications:
The modifications to the front block are the most extensive. Unaltered, the block is approximately 0.623”
thick. The block has to be modified so it is 0.473” thick. (The ranges of the 3 blocks made are from 0.470-
0.475”- mine is .0473” thick). The simplest method is to go to a gunsmith and ask how much it would cost
to mill the front block down to that thickness. The milling of the front block can be done with or without
the ramp assembly installed. The simplest method is to do it with the ramp assembly installed. This saves
an extra step of having to cut the ramp shorter with a hacksaw. Milling can run between $5.00 to $35.00
(also depends how well you know your FFL/gunsmith )
Re-read this next part as often as it takes to understand it CLEARLY. Being a setscrew retention method,
the mill operator will have to be very, very careful. There is not exact way to describe this or give proper
measurements. Milling takes place mostly, on the section that goes toward the barrel (or the 223 magazine
opening side). Tell the gunsmith/mill operator to mill as much material from that side as possible, so
as long as the holes and threads for the set screw are not compromised!!! Then, have the other side
milled to remove as much material as needed to make the 0.473” thickness. If done correctly, part of
the ramp “post” will be exposed, but not much. The ramp post on the JRW is closer to the magazine side
rather then the „barrel side‟.
The JRW block, as others, have this portion that narrower that allow it to fit inside the 223 magazine well.
The best way it can be described is a step down or a recessed portion to fit in the narrower part of the
magazine well. The raised portion of the block on the sides will have to be filed down until they are the
even with the recessed part. If this is not done, the block will not sit all the way forward. The hardest part
of filing is the anodizing of the aluminum. Getting through will be the hardest part. A Dremel with a
sharpening wheel may speed up removal, but final „smoothing‟ will have to be done with a file. (I
personally just filed away until it was the same height.) The front edge of the block will also have to be
beveled slightly. This allows the block to sit forward and not contact the corner of the magazine well.
After machining, the original feed ramp will have a fairly large “edge” on it. Use the files to put a new
„ramp‟. Usually filing a “V” notch is the simplest method. Take the round file and file a “groove” down
the middle, making it deeper at the bottom and getting shallow on the top. Next take a flat file and blend
the edges in with the rest of the ramp.
The photo below shows the ramp and how much filing was done. It also shows a better view of the
exposed ramp post.
Magazine Notching
Notching of a magazine can take place after the Sten block is installed in the rifle. The simplest method is
to push the magazine button in far and swing the magazine catch out of the way, exposing the “hole” in the
receiver. Insert a Sten magazine as far as it goes inside and hold it there. At this point, you can cycle the
bolt while holding the magazine in place to see if it will cycle freely. Don‟t push the magazine in too far
that it starts to stop the bolt from moving freely. When satisfied that the bolt will still move freely, mark
the magazine. With a small screwdriver or scratch awl, mark the outline of the catch hole on to the
magazine.
Move the magazine catch out of the way like this:
The mark on the magazine will look like:
After marking, remove the base plate and take out the spring and follower. Take the drill and drill bit (size
just under ¼”) and drill the curve out first, being careful to stay with in the line. Next, take the Dremel and
the reinforced cutoff blades and cut just a hair width away from the line. The metal can be eaten away with
the cutoff blade near the back of the magazine where it turns to the back. The lining will fall out when you
enough material have been removed. Clean up the magazine catch with the round file or the carbine cutter,
working slowly up to the line. Check the magazine for fit and if the catch will engage the new notch.
Install follower, spring and base plate. You have one magazine done, a few more to go.
Finished magazine:
Afterward
The use of Sten magazines in the Colt style AR solves the issue of high priced magazines. The retail cost
of the newer single piece Colt type blocks can be as low as $120-200. For some, the amount of money
invested in a JRW block, $120 retail, seems extremely high for a block that will be “hacked on”. Is a
block and modification (may cost up to $160) worth it? For some, no… but for others, it is.
Second Afterward: By HighlandMac
When cutting the notches for the Sten mags, I have found it is easiest to just insert the mag (without spring
and follower of course) with the bolt closed. Mark the mags as described above. This has worked for me
with really no problems. Be careful when cutting the topside of the notch, too much cut out will bring the
mag too low in the magwell. When creating my magwell adapter I left the “wings” on the front piece. I did
so because the front of the Sten mag tapers off a little and with the “wings” left in place I fell it supports the
mag better. All of the work to modify the 2 piece JRW magwell adapter was done by me with only a
Dremmel tool and simple hand files. You can do all the work on the magwell adapter in about 3-5 hours if
you put your mind to it. Magazine notch cutting, after I figured out the best way to do it, takes about
10minutes per mag using a highspeed carbide steel cutter in the Dremmel tool along with a cutting wheel. I
use no hand tools for notching the magazines. This has been an excellent project and will save you much
money over buying factory Colt mags!!
Good Luck!