IMO,its good that this topic is discussed here occasionally because there is a lot of wishful thinking out there.
I'm all in favor of people having good information to make good choices.
This topic has been discussed on TFL at length.
The short answer,regardless of state laws,the form 4473 is a Federal document,the BATF deals with Federal Firearms law,Marijuana is a Federal controlled substance.
Note,if you are shooting on Federal property,such as BLM land,National Forest Land,etc,the nice Ranger in the green truck may well be paid by the Feds.
BATF guidelines suggest if you have used within a year,you are a"User Of ".
A failed drug test documents you,as would aa MedPot card.
Your hair,and nail clippings carry your drug history.
In Colorado,it clearly states in the application for a Concealed Carry permit,a pot user is not eligible.
Whether or not you would be prosecuted is debatable,but remember an administration can be tolerant of recreational drug use at the same time it is hostile to firearms ownership.
Suppose you are in a car accident. (or a SD shoot). A toxicology report might be done. Trace of marijuana from two weeks ago is found.
Technically,a single round of 22 LR ammunition is a big problem.
Its a felony to lie on your 4473.
If anything proves you are a "user of" you are a"prohibited person"
and you are in possession of or transfer any firearm or ammunition,that would be felony number two.
Even in Colorado.
This will take you to one prior thread
http://thefiringline.com/forums/showthread.php?t=539207&highlight=marijuana+guns
And another
http://thefiringline.com/forums/showthread.php?t=506006&highlight=medical+marijuana
If you use the "Search" function you will find several more threads.
These are lengthy,detailed,and well supported by the legally well informed Staff.
Excerpts of applicable law are posted.
I think if you use search and read through the previous discussions,you will be very well informed.