If it was me doing it so as to make it so that it could never be returned to A or R fire, I'd remove the gas cylinder completely and machine out the threads in the body. Use the old gas cyl to manufacture a good copy of the original and PRESS FIT the new gas cyl into the body, as per the Mk2 gas cylinders. There, a solid gas cylinder pressed in and if you wanted to really impress the judge, drill into the gun body, just behind the blast shield and mig weld the new solid gas cyl to the main gun body.

Punch out the as block tapered pin and press the gas block off. Quite easy. Weld up, make good and assemble.

Would such a conversion be legal anywhere in the known universe. I can't see the word BOLT ACTION REN GUN featuring on a UKicon FAC any time soon. I recall tha total palava surrounding Cadet L98 SA80's that were MANUFACTURED as bolt actions. The Military police experts suggested that they knew better than the manufacturers....... But what I have suggested is exactly, in effect, what the L98 was!