The outer barrels aren't drilled out as far as I can see - just plain tubes so must be of the shelf (they wouldn't have had bits of steel tube especially made surely).
... so a correct OD tube with an insert at the muzzle end might be more practical.
Don't get what you mean by this.
Basically reproducing the AGM barrel.
A 16mm OD tube could be found with a suitable wall thickness to put a double-ended M14x1mm adaptor in the muzzle end.
Cheers for checking, C-W 
The new tracer units are different to the old TM one I used to have - they had a flat back with various adaptors for the different guns. I tried the Sten muzzle adaptor for size on a spare tracer adaptor and although it would need some filing out it could be do-able, but it would then necessitate using a TM tracer unit. I don't normally have the plastic muzzle on the Sten as it looks 'wrong' to me, I've padded the inner barrel with small 'O' rings to hold it in place and it looks fine, imo.
"A 16mm OD tube could be found with a suitable wall thickness to put a double-ended M14x1mm adaptor in the muzzle end."
There is a 14mm+ <> 14mm- adaptor available for MP5k's but by the time the barrels been sourced and threaded at the gun end (looks to be 16mm) and threaded at the muzzle end, which would then have to be 14mm+ for the adaptor, it would surely be easier to just make a 12mm<>14mm adaptor ???
Unless the existing barrel could be re-tapped from 12mm+ to 14mm+, or would there not be enough meat left on the barrel ?
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Unless the existing barrel could be re-tapped from 12mm+ to 14mm+, or would there not be enough meat left on the barrel ?
That's possible. The drill hole for an M14 x 1mm thread is 13mm. That'd completely remove the 12mm thread.
The thread would be 0.5mm deep all around, leaving about 1mm solid material around the outside of the thread.
Perhaps that's the way forward then ......... 
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Terms & Conditions of Sales :
Payment by Paypal only > Cleared funds must be available when deal is agreed - no e-Cheques.
Possible Swaps/Parts-Exs (with regular forum members only) -
* British BD Trousers & Blouse, to fit 48"-50" chest, 40"+ waist, tall fitting/long leg
* British Gaiters/Anklets, not blanco'd, largest size available
I've had a look, and there are various tubes available on Ebay, including this one:
http://www.ebay.co.uk/itm/16MM-OD-X-10M ... 337c35d645
That looks a bit like what the existing barrel is made of, and would support an M14x1 male thread.
In the morning, I'll properly measure my Sten barrel.
I've been over my Sten's barrel with a digital vernier.
The barrel outer diameter is 17.3 mm (0.68").
The inner diameter of the main tube is 10 mm. The inner diameter of the screw-in tip is 8.6mm (this supports the end of the inner barrel).
The thread at the "breach" end of the main barrel is 16.5mm diameter, and looks like 1mm pitch. It's possible it's a loose fit M17 x 1 mm.
The length of the main tube (excluding the muzzle "cap" is 170 mm, and the inner barrel muzzle is a couple of mm inside the end.
I'm going to see if I can source some suitable pipe.