Steel core pistol ammo accomplishes the same result, and doesn't require a new platform and stocking a new caliber. Is there any real benefit to 5.7x28 or 4.6x30 over steel core 9mm in an UZI, MP5 or any other SMG (or even a pistol)? If there is a benefit, I can't believe it outweighs the logistical benefits of not adding a new caliber to supply chains. While I understand that AP pistol ammo is unobtainable for civilians in the United States, but so are full house 5.7x28 and 4.6x30. Moreover, HK and FN are selling these to military and law enforcement who are not subject to these restrictions.