HK receivers seem to be made in China so the free trade provisions would not apply... but it is possible that the specific tariff code that receivers fall under has zero duty; I don't know.

Is it possible to gently ask why we think there is no duty on receivers ?

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8