This is the official South Australian Police email reply that i recieved this morning:

I have your enquiry for response….
Spud guns are “prescribed firearms” by virtue of Regulation 8, being home made firearms. There are heavy penalties in the Firearms Act for possession of a prescribed firearm ...

6000 N/m/m means that if you got a 1 meter long section of pipe, and clamped one end in a vice, the other end would deflect 1 meter if you if you put 600 kg on it.

Now, I expect a 1 meter section would actually snap before it deflected one meter, so let's just say that it'd "take 60 kilograms to ...

Seems i'm stuffed anyway, just went out and checked what i was going to use for the chamber. It's DN100 SN6 as layed out in AS/NZS 1260 (Australian / New Zealand standards). I.e. not pressure pipe, but according to this document http://www.vinidex.com.au/vinidex/live/RESOURCES/IMAGES/technotes/TN ...

y not just save some time and sleeve some pn18 pipe in another piece then wrap it in fiberglass it would be able to hold allot more pressure

Because what I specified in my original post is the best that I can get at this time :( . The results that were calculated using the spudtech spreadsheet ...

For the reservoir chamber on the gun I intend to build IF the S.A. gun laws allow it, i was thinking of laminating some 90 PSI PVC inside some of the same rating (i can get two diameters that will fit each other like a glove) and then wrapping the whole lot with several layers of fibreglass ...

this thread:
http://www.spudfiles.com/spudtech_archive/viewtopic.php?t=12043&postdays=0&postorder=asc&highlight=sprinkler+valve+mod&start=20

I have two of these. Now i intend to use this method on them. Forgive my ignorance, but if the diagonal hole is widened (speaking of the diagonal hole show in ...

Prescribed Firearms legislation:
http://www.legislation.sa.gov.au/LZ/C/R/FIREARMS%20REGULATIONS%201993.aspx

OK, thats the current south australian firearms legislation. I've emailed to confirm, but as far as i can tell, any sort of home-made firearm is a prescribed firearm. This is a good and a bad ...