Sadly only reproduction long-arm flintlocks are to also be considered antiques....

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Don't know what's meant by this, but I thought that the law was pretty clear regarding antiques. You just have to read it through completely and take the time to understand it. Surprisingly it really is one of the easiest to understand laws regarding firearms, there is no ambiguity or subjectivity etc.
Reproduction guns are only antiques if they are long guns and are flint match or wheel lock. That is it. Reproduction percussion cap long guns are not antiques, but you can go by a brand new flint lock made last week (or build one yourself - and it does not have to be even a reproduction of an antique gun. Any flint wheel or match lock rifle or shotgun is legally considered an antique.
Only original muzzle loading handguns, made in or before 1897, are antique. They can be flint match wheel lock or percussion cap, no matter.
For cartridge rifles, has to be bigger caliber than 8.3mm (which is about 32.67 caliber, or 0.3267 inches diameter bore) from land to land. Can fire modern cartridges as long as they are big enough (of course the gun has to be able to take the pressure, eliminating most modern cartridges anyways, though you certainly could hand load). Single shot only, no lever or any other sort of repeating gun. Basically if you do not have to load in each cartridge by hand then it is certainly not an antique. One point of confusion here for me, is I wonder if you Could take a sporterized/already bubba'd gun like a Winchester lever action, and remove or permanently alter it so you have to single load and can not repeat, then maybe it could be deemed an antique? Totally not sure on this, but technically, by the law, it doesn't say that anything about regardless of being modified for single shot fire etc, and normally they mention modification if they care, such as with antique cartridges for handguns. They can be modified to fire an antique cartridge and are then antique if they are old enough.
As for cartridge handguns, it is again very simple. If it is old enough, and does not shoot a caliber on the no go list, you are good and it is an antique. Commercial availability of the ammo doesn't matter. The one thing I'm not sure of is auto loaders (must be a few that are old enough maybe? Maybe not, don't honestly know. But I thought I remembered something somewhere about magazine fed handguns, though I can't find it now, so maybe I made it up!
Very easy imo.
Hope I didn't piss on anyone's corn flakes but I find it pretty simple. The hard part can be ensuring that a particular gun is in fact old enough, and proving this. One must be able to prove, with published documentation, or proof marks, or factory letter, etc (I.e. a reliable source, not just some guys on a forum saying it is old enough, not just some guys webpage. Must be published if you are going to use am article etc, and no Wikipedia probably wouldn't count). Then the next challenge becomes dealing with the cop or fish and game officer etc when you get "caught" legally enjoying your property on crown land or private property. You may have guns drawn on you, but staying calm and nice, and having all the paperwork and laws to back you up will get you allowed yup go free, it just might take up some of your time.
My personal biggest problem with shooting my Webley MkI is nothing above - it is simply finding crown land to shoot it on. I've got a private farm I can go to but it's a drive.... I'm trying to find somewhere a bit closer I can take a buddy or two out to and get him into shooting by shooting a handgun.