There are some super minor differences in nif formats, but nothing that will effect usability. But I suspect that the SightingNode entry in Nifskope is what controls the iron sight aiming/animation. I wish I knew exactly how to do this, but I haven't tried setting up a weapon for FONV yet. Also, I think the weapons need to be set up a little differently to work with the new iron sight aiming system. Most custom weapons created for FO3 as mods probably made use of existing animations within FO3, and I'm sure some of those animations are not the same or do not exist in FONV. I don't think this is true for FO3 weapon models.