Many of us have been required to submit to drug testing to get or keep a job.It is a pass/fail test.One test is the hair test.If you have smoked pot during growing a hair,it can be detected.
If a person were involved in a legitimate self defense shooting it may very well be there would be tests performed to see if the shooter was under the influence of anything.Testing positive for THC would give the prosecutor a strong card to play to the jury regarding whether judgement was impaired when the trigger was pulled.
But,that is just problem one.
Problem two,this person is now proved a prohibited person in posession of a firearm,felony.
Problem three,if this person signed a 4473,false statement.Felony.
Consider if a person is hunting,fishing,shooting,there is a reasonable chance of being checked out by an LEO,such as a game warden.The question"Do you have any weapons? May come up.Red eyes,odor,a seed ,etc can lead to a search.
The OP had a great idea! Find out what the law is,where do I stand,how can I make good choices.
To take a position"Thats all BS" well,a person can do that.It might be difficult to get a judge to agree.