From the stickie above:
Note about various types of XM193: You may notice that XM193 has several different types in the ammo review below. There are basically what I would consider two main designations for this ammo:
Boxed. The boxed ammo may be designated XM193, XM193A, XM193C, etc. According to Federal, there are no real differences between these types: "All XM193 is the same. The letter designation after the XM193 is only for packaging designations. They all go through the same loader, inspection process and are all the same quality of ammunition no matter the suffix." In the review below, please check the "Federal/LC XM193" review for all versions of boxed XM193.
Loose-packed. In the 2005 time frame, loose packed ammo was being sold on the market as XM193PD (and the XM855PD equivalent). It was quite apparent that this ammo was much below the standard of the boxed ammo: Split necks, loose bullets, severely dented cases, etc. As of early 2009, this ammo has not been available for several years. XM193BK has recently surfaced on the market, but there's not enough known about it yet to see if it's more like XM193PD in quality, or if it's different enough to warrant its own review.