From scratch I mean,
from what little info I've been able to gather it's okay if it's not fully automatic, not being made to be sold (Without a license) or emulates guns that are illegal. But all this info seems to be on premade gun kits.
I dunno, just would be interesting to make a revolver that shoots shotgun slugs or something.
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I think it is unless you get caught. I'm no legal expert though.