I personally would apply for a permit. They are, mmm were, easy to get. Being an air rifle rear sight will probably not gel at Customs. It's a firearm sight to them.
Same as having to get a permit for a scope. You could argue that its intended use is for an air rifle. They won't see it that way.
And if you do apply and your permit is "declined" (cop speak for "not required") due to no permit required, you have a bit of paper all nicely signed off to present to customs if things do turn to custard.
The declined permit they sent me, all nicely signed off by a senior person a Kapiti, I have sat on in case I do get a customs knock back sometime in the future.
That incidentally was earlier on when they started enforcing the permit requirement and there a lot of confusion around the is and is not required, and was for reloading dies.
Bookmarks