Pretty well as per thread 32. 1) Machine thread the Mk2 body to accept the Mk1 or 3 threaded type gas cylinder or 2) Machine off the approx .8" long threaded part of the plentiful Mk1 or 3 cylinder to make an undersize diameter. There's plenty of meat there..... Press on a high chrome* steel collar to increase the size back to the approx .9 or so standard dia and press in then align to suit**.
*The collar could be made from any suitable higher than usual carbon steel and in any case, it ain't goin nowhere as it's retained at the rear by the start of the gun body shoulder and at the front, by the larger dia shoulder that it's pressed against. Cylinder is pressed in place, gun is ready to go
** we had a special gauge to align the gas cylinders but a good engineer or Armourer could align it by eye using a suitable barrel. Forget the grief and aggro that we had to go through reaming the tapered hole........... Drill out and press in an unshakeable roll pin
Just my thoughts as someone who might soon be doing loads of 'em!
As you'll quickly realise from this thread, I don't belong to the 'where there's a will, there's a way' school of thought but from the slightly similar 'where there's a will, there's an EASY way' school of thought.......