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We modified the receivers, easiest conversion ever.
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The easiest conversion is to buy a FA hammer pack. Seconded by making the SA hammer pack FA by swapping some parts. Modifying the receiver requires a FA stock. FA parts kits aren’t exactly inexpensive.
I would personally not convert the factory receiver, because it forever removes it from ownership by non-SOT/gov/LE. One day the PS90 won’t be imported any longer, will be worth big money like pre-89 HKs, SIG 55x, FNCs, FAMAS, etc. And you’ll likely wish you could keep it if you give up your SOT, or wish you could sell it to a regular collector on the commercial market.