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The Gatalog presents:
How to add DIY Barrel Feed Cones to ECM Barrels
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Have a look at this release -- it's a tutorial on doing up a feed cone. I'm not sure if there are instructions in it for adapting this to an FGC-9, but if there are, this'd be your best bet:
https://guncadindex.com/detail/Barrel_Feed_Cone_Tutorial:a