You do not need an invite OR hunting lic to apply for BATF 6NIA but you DO need an invite when crossing the border, a hunting lic will NOT cut the mustard with the US Customs people. Not sure if BATF has new forms up or not yet. If forms still have Invite or Lic requirements, I would phone them and ask if they are still required.
I handed in both the Hunt Lic and Invite when heading for US Nats and they told me outright that they denied my entry based on the Hunt Lic but did grant entry based on my invite. Ya, I know, US Customs are F'd. All the hubbub for no reason, guess they like the drama. (This was at Osoyoos crossing, never had an issue crossing at Sumas)