PTK's correct. Machineguns require a $200 tax stamp to transfer. After 1986, no machineguns can be manufactured or imported, so there's a fixed supply in the country. What makes the Mac's nice is that the lowers are the firearm and there are a lot of accessories and uppers available. It's kind of like the AR of the machinegun world.
Mine cost around $3400 and then I paid extra to have the Uzi grip put on it. The folding stock, foregrip and upper came from Lage and was around $1200 if I remember correctly.
The original Macs used plastic Zytel mags for the most part which are not cheap and are pretty crappy from what I've read. Many people do a sten conversion which allows the use of sten mags. The downside with sten mags (as I understand it) is that they are double stack, single feed. So they're not all that easy to load. The Uzi mags are double stack, double feed, so you can just push the round down on the top of the mag and it clicks into place. No pushing down and sliding the round in to load it. I got 9 Uzi mags (32 round magazines) for about $90 bucks or so. About ten bucks per mag, pretty cheap. They're steel, sturdy and plentiful. I got my Mac and uzi conversion from Sam at Practical Solutions. He also did the work on the Lage upper so it would work with the Uzi grip.
Blakenzy- I should be able to shoot it with the stock closed. The charging handle reciprocates as it fires, but there is space between it and the top of the folded stock. It actually touched at first, but you can adjust the stock and I moved it so it was slightly lower and left some space.
Can't wait to shoot it full auto!
Shawn