I was wondering why federal's military ammo like XM193 ball or XM856 tracer has the "XM" prefix instead of the normal "M" prefix that adopted military ammo has. I thought they were the same rounds.

I'm not an expert, but I think it's because they are LOTS rejected by the military so to avoid confusion they use an X prefix.  It's still M193 or M885 but it's just rounds rejected by the military due to dents in cases or sealant problems.  It's still very good stuff though.

Quoted: No idea. It's commercial ammo, so Federal can call it anything they want. <shrug>

You sir owe me a keyboard and a new cup of coffee.

Quoted: I'm not an expert, but I think it's because they are LOTS rejected by the military so to avoid confusion they use an X prefix.  It's still M193 or M885 but it's just rounds rejected by the military due to dents in cases or sealant problems.  It's still very good stuff though.

From what I gather, this is pretty much what it seems the explanation is.

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

Most of the designations are for internal purposes at ATK. The X generally denotes product packaged for the commercial market after having been released from a Mil or LE contract order.

For example XM193, XM1911, XM33, XM762, XM856, etc are military contract overrun. XMAE223SP is from an ICE contract. There are several XM40 loads from LE agencies, and so on.

Quoted: Most of the designations are for internal purposes at ATK. The X generally denotes product packaged for the commercial market after having been released from a Mil or LE contract order. For example XM193, XM1911, XM33, XM762, XM856, etc are military contract overrun. XMAE223SP is from an ICE contract. There are several XM40 loads from LE agencies, and so on.

whats and ICE contract? and what was the xm40 load?

Quoted: Quoted: Most of the designations are for internal purposes at ATK. The X generally denotes product packaged for the commercial market after having been released from a Mil or LE contract order. For example XM193, XM1911, XM33, XM762, XM856, etc are military contract overrun. XMAE223SP is from an ICE contract. There are several XM40 loads from LE agencies, and so on. whats and ICE contract? and what was the xm40 load?

Ammunition contract for The Bureau of Immigration and Customs Enforcement.
There are several .40 loads XM40LP1, XM40HA, XM40HB, etc. They are built to requirement for different agencies so load information varies.

Ammo Oracle