The basic Maadi-Griffin came out in the late '90s, either assembled or as a kit. 50 BMG. Oh, there was a pistol version too... the guy who was making them ran afoul of the ATF for unrelated matters (starting with selling an illegally manufactured machine gun to an ATF agent) and went to the Big House.
Anyway, it's basically a tube gun, very slick design. The bolt cap is also the buttplate; you twist the buttplate and pull out the bolt and the spent cartridge. There are 35 parts, but most of those are screws, pins, or springs - the basics are a couple of pieces of square tubing, some muffler pipe, and a foot or so of 2" diameter 4140 steel.
They sold a booklet with plans and instructions on how to make one. It's not for sale any more, but you can get it via the torrents or on usenet. It's a low-quality scan, but readable.
Oddly, there aren't many pictures of the gun on the net; most of those are the "pistol", of which probably one or two were made... supposedly 300 complete rifles and several thousand kits were sold before the company folded.
Whoever did the original design was on the ball. It would help to have a lathe that could chuck a 2" workpiece, but the rest could be done with a mini-mill. If you've looked at any of the other .50 plans out there, the Maadi-Griffin is full of "what didn't I think of that?" ideas.
First time I saw one, I WANTED one. But BMG barrels had gone from $50 to $300+, and I really couldn't justify it from my limited entertainment funds. Besides, a friend has a Barrett, and I can shoot it if I want.
Then there's Ed Hubel, who started with "first you take a .50 BMG, then you blow it out straight..." And I wanted one of those, too. And then the Voices said, "why not save the gunrack space, and combine them into one?" And so things rested, until I saw that barrel on Gunbroker.