The main problem with counterfeited parts in any device is the probability of them being so far out of tolerance so as to make the device fail and cause harm to the device user or his property. I don't think the tolerances on firearms are nearly so critical as they are on, for example, aircraft parts. If the part is so far out of tolerance basically the firearm won't work at all. Failure causing harm by firing the firearm will probably be obvious before it can be fired in most cases. So, if the firearm works fine, the fact there may be a counterfeit part in it, isn't that big of a deal as with something like an aircraft part, I don't think.
Any part replaced by the user that isn't a part made or certified by the original manufacturer might be considered "counterfeit", so the concept of counterfeit parts in a firearm is kind of a "what-if" exercise anyway.