For the benefit of others who might be contemplating this, not just Jaba, I will put a brief run-down up!
Back in the day, a few days before flight, I used to just pitch up at the local field office, and ask for a letter granting permission, which they called an instrument. I did this on a flight by flight basis, no problems, they just changed the details and hit print, took about 2-3 minutes and cost nothing.
Then, they changed it...
They started doing 'blanket approvals', for a fixed amount of time, rather than individual flights, say this instrument covers all flights, by you, from 1 January 2014 to 30 June 2014, and it had a bunch of conditions. Very much like the picture already posted. Still took about 2-3 minutes.
Now, as of about 3-4 years ago, rather than simple and effective process that it was, you need to WRITE to your local office (email is fine) more than 2 weeks in advance, to have an instrument drawn up, which can cover a block approval, for a specific PIC, for anything up to a 2 year period. This will then be available for collection at your local office, AFTER PAYING A FEE OF ABOUT $219. It is still the same 2 page document, but now they want to add their 'minimum charge' for doing it. They tell me it takes 45 minutes, and they only charge in minimum time, but I suspect they still just go to your file, change the dates of the previous one, click print and sign it. Thieves. So at least make it worth your while and get it for as long as possible, and for any purpose, not just the short time and specific purpose like the example in the above picture.
Or, you could just chuck it in with your underwear down the bottom of the bag and nobody would be any the wiser, because I am reasonably sure no FOI/AWI would be looking through your luggage (nor would they have any legal justification to do so? But I stand to be corrected on that point) at any ramp check...
That point aside, when you do have your magic bit of paper, I think mine also said that they should be inaccessible in flight (ie, in the nose locker, if you have one!) and ammo should also be stored in a separate compartment if possible, and also inaccessible in flight, if possible.