It would need to be a lot more permanent that just welding up the gas port. It is unlikely that the Police would accept any modification at all that would change an AKM to A category, as they maintain (rightly or wrongly) that the parent weapon always remains and that if it could be modified then it could also be rebuilt. The way the Arms Act is worded, their point of view has some validity. The definition of a 'firearm' in the Act includes the following "Anything which is not for the time being capable of discharging any shot, bullet, missile, or other projectile but which, by its completion or the replacement of any component part or parts or the correction or repair of any defect or defects, would be a firearm....". That is a wide open interpretation and any modification of an AKM to straight-pull would be unlikely to escape it.
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