Quoted from TGZ conversation with ATK.
TGZ: Please explain the nomenclature (i.e., the "XM" prefix and the "PD" suffix), and also explain how XM193, XM855-PD, XM118LR-PD and similar Lake City / ATK ammunition differs from milspec (i.e., M193, M855, M118LR) ammunition, so we may present the correct information to our readers.
ATK: We've had several questions regarding the XM designation and following is our official statement regarding what the product is.
XM193 ammunition is 5.56mm contract overrun material. It may not meet all of the mil-spec requirements, however, it does meet all requirements of commercial ammunition for pressure, form, fit and function.
As far as the "PD" suffix, it simply denotes bulk packaging.
TGZ: Why is it that the XM ammo "may not meet all of the mil-spec requirements?" This seems to be the most common question on people's minds.
ATK: I don't know all the ins and outs of the mil-spec requirements, but like any other mass-produced product, there are often imperfections in either appearance or non-functional mechanics. As we all know, the military is very stringent in their requirements and for good reason. This being the case, it doesn't take much to fall outside of those parameters.
TGZ: Thank you very much. This should help put a stop to some of the more outlandish speculation and rumor-mongering.
The response from ATK came from Jason Nash, Group Lead, Communications - ATK Ammunition and Related Products Group.
http://www.atk.com/customer_solutions_armamentsystems/cs_as_fm_lcaap.asp