The relevant bit(s) would be "It shall be unlawful for any person [...] who is an unlawful user of or addicted to any controlled substance (as defined in section 102 of the Controlled Substances Act (21 U.S.C. 802)) [...] to ship or transport in interstate or foreign commerce, or possess in or affecting commerce, any firearm or ammunition; or to receive any firearm or ammunition which has been shipped or transported in interstate or foreign commerce."
So a prosecutor would need to prove that I was "an unlawful user of or addicted to any controlled substance" at the time that I received a firearm or ammunition that has crossed my state's borders (which would presumably be nearly all the firearms and ammunition I own, but still). There's also the parallel prohibition on the seller's end (and another one involving the actual background check questions), with the same present-tense language of "is an unlawful user of or addicted to any controlled substance".
> If third party hearsay is good enough for a warrant why wouldn't video evidence of you at the gun store and the pot shop?
A warrant != a conviction. The video evidence of me at both stores would not establish that I was "an unlawful user of or addicted to any controlled substance" at the time that I took possession of the firearm.
Now, if there was video evidence of me smoking a phat blunt right outside the gun store before walking in and walking back out with a firearm, then yeah, that'd be a slam dunk case (assuming the prosecutor could convince a jury that it was indeed cannabis I was smoking). Likewise, if I was actually tested for cannabis in my system and the tests indicated that it was indeed in my system at the time I took possession of the firearm, then that would similarly be a slam dunk case.
(I guess even in the latter case I could maybe argue "yeah, I was an unlawful user, but I quit before buying the gun", but that defense is pretty weak if it's still in my system; I reckon it'd be stronger if I was sober for an actually-significant amount of time.)
Barring those? I ain't a lawyer, either, but my personal understanding is that the present-tense wording leaves a loophole big enough to drive a truck through it. Obviously the safest bet is to not rely on that being the case, but it's pretty easy to answer "No" to "Are you an illegal user of or addicted to any controlled substance?" without it being a felonious lie.