'20rnd' Thompson Lo-Cap Mags - A Conversion Guide

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'20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby McVickers » Wed Apr 22, 2009 12:52 pm

The desire to have '20rnd' length mags' in a Thompson, especially if you're doing a Brit impression, may have come across your mind - So for all of those that want to use 20rnd sized mags in their Thompson, but are put off by the price of the TM high-cap ones, the lack of cheaper AGM alternatives or the rules enforced by WW2 themed games (e.g: GMA's ban on all hi-cap/rattling mags); then this might be up your street! They hold a usable 40 BBs (41/42 in total) only 7 less than the advertised capacity of the '30rnd' mag's before conversion.

1. First, take a Cyma/Cybergun brand '30rnd' lo-cap (the type that comes with the gun). These mags' are identifiable by the three screws down one side and the lack of detail on the base of the mag'. Open the mag' by unscrewing the three screws and pulling apart the body halves (The internal spring may cause parts to fly out, BTW). Take out all of the internals and put them safely to one side.

2. Re-assemble the mag' body halves together minus the internals and mark them up with two lines (seen in the picture as edges of marking tape) - one just below the 15rnd marker dot/window and one just above the 25rnd marker dot/window. Cut the mag' into three sections; squarely along these lines:
http://h1.ripway.com/Mansunite/Pictures/01MarkUp.JPG

3. Once cut, check that the cut was square and true. Do this against known flat perpendicular surfaces for the top and bottom sections of cut mag'. Throw the middle section in the bin:
http://h1.ripway.com/Mansunite/Pictures/02Cut.JPG

4. Using a known flat surfaces, sand the newly cut edges of the top and bottom pieces of mag' to ensure a flat edge (especially if the cutting wasn't precise enough), and to reduce the cut edge down to the level of half-way through the marker dot/window nearest to the cut on the sides of both top and bottom sections of the mag:
http://h1.ripway.com/Mansunite/Pictures/03Sand.JPG

5. Once both cut edges are sanded down to this level, the top and bottom section of mag' should mate nicely with each other. Note: The two halved marker dots/windows now match up to produce a singular whole marker dot/window, which is the new 15rnd marker dot/window on the '20rnd' mag' - if they do not, and instead look a bit like a figure-of-8 or an oval, then sand down the edges some more. Using the position of the marker dot/window this way helped me guarantee the length of the converted mags' produced were correct and all would be the same length. Disassemble the mag' body halves again and glue the relative top and bottom sections of each half together with a smear of superglue:
http://h1.ripway.com/Mansunite/Pictures/04Glue.JPG

6. Once the superglue has dried on both newly made '20rnd' mag' body halves re-enforce the joint internally with some glued in strips of Plasticard or PVC sheet and as much epoxy as you desire. Just don't put any glue in the 'U' shaped BB channel! I re-enforced mine with plastic strips and epoxy in the base of the metal weight recess (leaving space still for the inclusion of a weight to be fitted) and inside the mag's insertion guide/rail.

7. With the top and bottom sections now glued and dried, sand down each body half and fill with epoxy putty to hide the join. Keep fitting the two body halves back together every now and then (as shown in the picture) to ensure the two halves haven't warped to miss aligned during the glueing/sanding/filling. Sand down the epoxy filler putty when dry to a smooth flat level:
http://h1.ripway.com/Mansunite/Pictures/05Fill.JPG

8. Lightly sand the whole mag' body to prepare it for a bit of paint. Mask the top end and feed hole with some masking tape and block up the screw holes on the side with a bit of bluetac or similar:
http://h1.ripway.com/Mansunite/Pictures/06Prepare.JPG
http://h1.ripway.com/Mansunite/Pictures/07Prepare.JPG

9. Paint the entire mag' in a series of light coats. I used a cheap but hard-wearing can of black automotive spray paint bought from a £1 store. It's not quite matt and it's not quite gloss, and so looks really rather good and looks more realistic than just the bare Cyma plastic.
http://h1.ripway.com/Mansunite/Pictures/08Paint.JPG
http://h1.ripway.com/Mansunite/Pictures/09Paint.JPG

10. Once the halves are fully painted and dry, reassemble the whole mag' with it's internals. When reassembling, you've got to cut 10cm off of the internal spring for it to fit and operate nicely and make sure the U-shaped BB channel on both halves is smooth and clean. You'll only be able to fit one of the metal weights back inside the mag', due to the reduction of the mags' length, so add a little silicone adhesive or foam packing to either end to stop it sliding about in it's now oversized recess if it is a loose fit - but most sit tightly enough to not need this.

Further alterations to consider could be:
(a) Cutting the internal spring guide down a bit now that the spring has been shortened - perhaps manage an extra 5 BBs worth of capacity per mag'?
(b) Reassemble the mag with it's internals before filling, sanding and painting. Thus allowing you to eliminate all joining seams and fill up any screw heads showing.
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby CHThree » Mon Apr 27, 2009 8:07 am

I like the way your thinking! My 190 Rd hi caps rattled like hell yesterday at Die Glock, especially as I was only able to half fill one of them but they are all I have.
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby Steiner » Mon Apr 27, 2009 8:39 am

Excellenty-written guide, McV. :good: How long did it take you to do the work?
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby McVickers » Mon Apr 27, 2009 12:38 pm

EDIT: Just so people know, these are the mags' I use in the conversion - available from RSOV.com quite cheaply for 2: http://rsov.com/index.php?target=products&product_id=1682

Steiner wrote:Excellenty-written guide, McV. :good: How long did it take you to do the work?


Thanks :) Glad it's useful/of interest.
With all of the parts handy, the entire work only takes about 30mins per mag; a couple on mins' to cut the mags' (I recommend using a hand-power mitre saw), about 5mins to prepare, line-up and stick the halves together, and the rest of the time sanding the cut edges down to the correct height and 'flatness'. The real time-ache, is the waiting for superglue to dry, epoxy glue to dry, epoxy filler to dry and paint to dry! With that included, it takes about 2-3hours to get to the final coat of paint and then they can be left overnight for the paint to harden-up. Ideally, which I've now started doing, is cutting all my mags' in a batch, sanding them all down together, gluing and epoxying them all at the same time, etc... Saves on time, mess and equipment/tool usage and storage :good:
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby Waldiebeast » Tue May 24, 2011 10:48 am

If you were to use the hi cap mag, all you would have to do is strip it down and realise that there is a large area below the mechanism. You could shave that down, put back on the floor plate and voila. Hope this helped :good:
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby No1_sonuk » Tue May 24, 2011 11:44 am

Waldiebeast wrote:If you were to use the hi cap mag, all you would have to do is strip it down and realise that there is a large area below the mechanism. You could shave that down, put back on the floor plate and voila. Hope this helped :good:

Short hi-caps are readily available already. There's no need to make any.
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby rammix » Tue May 24, 2011 11:56 am

do you a link to any shortcaps?? :good:
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby No1_sonuk » Tue May 24, 2011 12:02 pm

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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby McVickers » Tue May 24, 2011 12:13 pm

No1_sonuk wrote:
Waldiebeast wrote:If you were to use the hi cap mag, all you would have to do is strip it down and realise that there is a large area below the mechanism. You could shave that down, put back on the floor plate and voila. Hope this helped :good:

Short hi-caps are readily available already. There's no need to make any.


Yes, as No1-Sonuk has said, this conversion guide given to make 20rnd length Thomson mags' which arn't hi-caps, and quite a few WW2 airsoft events don't like the use of hi-caps (and the rattle annoys a lot of players too!).

EDITED: for spelling's sake.
Last edited by McVickers on Tue May 24, 2011 12:18 pm, edited 1 time in total.
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby rammix » Tue May 24, 2011 12:14 pm

[url="http://www.ukapu.org.uk"]Image[/url]

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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby shellshokt » Tue Jun 21, 2011 1:15 pm

This guide is great and I am going to convert my 3 lo caps, however I do have 2 'long' hi caps I would like to convert as well. I know that 190 round hi caps are available but I don't really want to buy any more magazines. Has anybody done a conversion on a 'long' hi cap that they would be willing to share?
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby No1_sonuk » Tue Jun 21, 2011 2:00 pm

TBH, you'd probably be better off selling the long ones and buying short. Much less hassle unless you're good with sheet metal work...
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby shellshokt » Wed Jun 22, 2011 6:41 pm

Thought I'd give it a go anyway, quite simple in the end.
Push out the two pins and remove the base plate and the plastic bit inside is actually in 3 bits. Unscrew the middle portion of the innards and then connect the top to the bottom, presto, innards are now 20 round length.
The fussy bit was cutting and bending the metal mag outer but all in all took 30 mins and I am quite pleased with mesself :happyswing:
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I now have 3x 20 round size low cap mags and 2 x20 round size mid cap (190 round ) mags
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby rammix » Wed Jun 22, 2011 6:58 pm

wonder if it would work with hi-caps?? can use em in my original 5 mag holder then :)
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WE P08 Luger 4"
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Re: '20rnd' Thompson Lo-Cap Mags - A Conversion Guide

Postby No1_sonuk » Wed Jun 22, 2011 7:50 pm

rammix wrote:wonder if it would work with hi-caps?? can use em in my original 5 mag holder then :)

That's what he's done. He shortened a 400-round hi-cap into a 190.

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