Wildcats.

[SG500]

I wouldn't think there is anyone out there that seriously considers that a 50 cal necked down to .17 is going to work too well.
Attached a photo of various silly ones that someone made up for fun.
50 cal on the left for scale.
I'm not an expert in this sort small calibre stuff but would think headstamps are the all important thing.
All inert, all projectiles are standard prac or lead filled.
Dave.

 

[raymeketa]

The 50 BMG case necked down to .375 caliber (375 Mach V) has been tried in competition and it is about the smallest diameter bullet that is practical. Smaller bullets can be made to work but only by shortening the original case. Most of those shown are simply gimmiks or joke cartridges but it's suprising how many guys will pay high prices to have one of them. They were most likely made in a lathe or with hand-made forming dies and never fired in an actual rifle.

Ray

 

[SG500]

The 50 BMG case necked down to .375 caliber (375 Mach V) has been tried in competition and it is about the smallest diameter bullet that is practical. Smaller bullets can be made to work but only by shortening the original case. Most of those shown are simply gimmiks or joke cartridges but it's suprising how many guys will pay high prices to have one of them. They were most likely made in a lathe or with hand-made forming dies and never fired in an actual rifle.

Ray

Its surprising how many fake Fatmac cartridges are out there, of course being necked down and shortened Vulcans there's no complication with headstamps...........

Dave.