Huh, very interesting. Those would definitely be deadly (considering that even "blanks" can be). I think the cartridge body might be plastic too, if I am reading that page correctly (probably not too odd, that is how shotgun shells are often made I think).

Of course those rounds don't have enough recoil to cycle a semi-automatic rifle so I'm not sure how much of an advantage having a high-capacity magazine for those would really give anybody.