FOUND THE MOTHER LODE.
I knew I had not imagined it.
It was Stefan (Bugstomper) who nailed this I believe (maybe he can chime in and confirm that all details are correct). Here is the link: http://forum.alienslegacy.com/viewtopic.php?f=3&t=9737&start=25
I have also added images and info that Stefan had posted below, in the hope that will all info gathered in one place we can archive this post as the definitive PR Hero Barrel guide:
Stefan based his work on the picture of the Barrel Extension sold by the Propstore (the same as provided by Harry further up this thread), and made this overlay:
Based on the fact that the thread had to fit into the Thompson barrel Stefan theorised that the only metric thread fitting that would be possible was an M14x2. He then theorised that the outer diameter of the barrel extension would appear to be approx. 18mm
He noted that the Propstore pictures makes it look like the extension is made of two parts:
- A tube with an outer diameter of 18mm and and inner diameter of 14mm
- A hollow 'screw' that holds the extension tube in place and at the same time acts as a restrictor to create enough gas pressure to cycle the bolt
He drew the whole thing up as a CAD using the comparison of a standard Thompson barrel at the top of the page with the modified and extended barrel at the bottom of the page:
AND then he built it...
Stefan's Barrel cage is built from 3mm thick steel, which started life as 40mm x 40mm steel box section tubing (which unlike aluminium squared tubing is slight rounded, not truly square). This is then machined, before having two sides bent in towards each other to form the triangle.
As he notes the bend inwards does not start in the corners of the square tubing, it actually starts further up, where the Barrel cage has been machined, at the bottom of the milled slots (the weakest point). If you look at the original vents you can see the same effect.
This is the 10 hole vent/triangle. The sides the 8 hole triangle are shaped differently (straight) as the slot between the triangle and the vent is milled down to the bottom as you will see from this CAD that Stefan created:
His assessment was that bending the part is actually quite easy for the front part. Just a vice needed to clamp down the bottom to keep it straight. This is especially important for the rear part as the material is not weakened by the cutouts. Luckily, you don't need to bend it much there due to the taper of the barrel. Nevertheless, he had to heat that area with a blowtorch to get it done.
So... Bugstomper... Firstly a million thanks and secondly... can you confirm all of the above? Where any modifications/changes missed? How far did you get with the 8-holer?
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