Two homebuilt pistols were seized among other firearms on Tuesday by police in Sydney. They appear to have plastic (or perhaps aluminum) lower receivers with angular magazine wells. Both examples appear very well made.
To recap, a number of improvised submachine guns have been seized in Australia recently, many based on the Expedient Homemade Firearms series of manuals by British author Philip Luty.
Various .22lr SMGs:
It would be uncivilized not to link once again to a test fire video: