The new 6NIA form doesn't require invitation letter or hunting license for Canadian citizen.
http://www.losttarget.com/form6.html
It depends on if you require a VISA to travel to the states.
VISA NOT REQUIRED
Legal Nonimmigrants Aliens who don't require a VISA
- As of July 2012 Nonimmigrants who don't require a VISA ARE NOT required to submit documentation (Hunting License / Shoot Invitation) with ATF Form 6NIA when submitting for approval.
* Some crossings may insist on a hunting license or shoot invitation. You can ask to speak to a supervisor but it may not get you anywhere.
May use or rent firearms and ammo without reservation – although purchase of a firearm (take possession) allowed only if they also qualify as a state “resident”.
VISA REQUIRED
Nonimmigrants Aliens who require a Visa
- Shoot invitation or state side hunting license (any state) is required to be submited with the ATF Form 6NIA application.
- Contact shoot management to receive a Shoot Invitation.
- An approved ATF Form 6NIA with stateside hunting license (any state), gives unlimited entry for valid reasons, not necessarily hunting.
Hunting license option is recommended for club visits, practice, attending shoots; league, fun, charity, without the need of a shoot invitation.
License Accepted: Non-resident shooting preserve, non-resident small game hunting, or non-resident non-game.