Some guns being imported are allowed to come in because they are unable to accept high capacity ammunition feeding devices. To alter the rifle, as it is with an SKS, to accept a HCAFD would be illegal since you would be creating a non-importable weapon. HOWEVER, ATF has repeatedly allowed pre-ban high cap mags to be modified to fit a number of firearms. So, while it may not be legal to modify the firearm, it is legal to modify the perviously existing hi cap mag.
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