Is your Mk1 a runner Joe?

As for the CPU's parts Bren I'd say that the 'gas cylinder' re-weld/repair is FAR simpler/easier to repair than a barrel nut area repair. I say this because the gas cylinder where it is cut isn't actually the gas cylinder per-se at all as the actual gas cylinder is only the first 3" (from the front) of the gun. It is in this first 3" or so, within the actual gas cylinder that the gas has imparted all the energy that it's going to, to the face of the piston as by then, the bullet has long left the bore and there ain't no more gas. In fact, just another inch or so to the rear are gas vents and once the face of the piston has come level with the most forward of these, then the gas in the barrel, gas cylinder and remainder of the tube have equalised to atmosphere.

You could call the round area behind the actual gas cylinder 'a round tunnel like tube section to guide the piston back into the gas cylinder' as that's all it does. The fit of the piston in that part is what I'd describe as a sausage in the high street!

If I had to repair this I wouldn't be too accurate about alignment. What IS important here is DISTANCE and radial alignment for the barrel locator and gas cylinder spigot.

I haven't had to do one of course