If looking to have a detuned 54 Airking. I would have a look for a 22mm piston kit and a weaker spring. The 12 fpe UK market 54’s use a restrictor washer in combination with the same spring I currently have in my rifle. .115 wire , rather than the .130 with in the FAC models. I believe the spring length is also shorter in both the factory detuned models 7.5 J and the 12fpe . I was able to disassemble without a spring compressor and it had about 1” of preload.
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