Clearly you want to find new production ammo. Again, surplus is great stuff for practice and fun but for "serious" ammo you will want to find ammo that's less than 36 months old.
In the M193 class it's pretty generally agreed that the best manufactured ammo is from the Lake City Army Ammunition Plant located in Independence, Missouri. 2000 manufacture Lake City (LC'00) and 2001 Lake City (LC'01) is outstanding ammo. It's assembled at the Lake City plant and boxed up by Federal. It is accurate for M193, is loaded quite hot and has great velocity as a result and it seems flawlessly reliable. Lake City ammo also has a reputation for storability and reliability. Several AR15.com members have tested 15 year old Lake City and it often turns out to perform better than newly manufactured commercial ammunition. It's very much the gold standard of M193.
Winchester Q3131 and Winchester Q3131A are considered close seconds, perhaps indistinguishable seconds to Lake City M193. Both are Mil-Spec M193 but many AR15.com members have reported that LC is loaded a bit hotter.
Q3131 is the U.S.-manufactured Winchester M193, but since 2000 (and coinciding with the transition at the Lake City plant which left it shut down), Winchester's M193/Q3131 ammo has all gone to the military. Due to the demand during the Y2K scare, Winchester had subcontracted some of its civilian M193 production to IMI. Winchester has continued this contract, and the IMI-produced ammo is labeled Q3131A by Winchester.
Q3131A also is somewhat famous for its shining new cases. Lake City kept their production costs down by not polishing their brass before shipping. Since the military was their primary customer why bother making the rounds "pretty?" LC often has spots and other material on their cases (not that this seems to impact its stellar performance at all). Q3131A is much prettier looking, though still not as sparkling clean as commercial ammo that's been tumbled clean. XM193 also comes in boxes with plastic spacers. Many people find these annoying.
As you can see, the complaints about the two types of ammo are so trivial as to be almost not worth considering.
(courtesy of Dave_G)
Alliant Techsystems assumed control of operations at Lake City AAP on April 3, 2000. There was a lot of stuff laying around in the warehouses, components, loaded ammunition and what have you, that had been produced under Olin's operations. Alliant wanted to get rid of all Olin materials, so they approached Federal. About September, several ammunition distributors were approached by Federal and offered the XM193 5.56mm ammunition, produced in 2001. Millions of rounds were sold, packaged in generic white boxes with the Federal logo on them.
Some of this ammunition may have been Olin-produced loaded ammunition of 2000 vintage that was never packaged and shipped. It was just stored in those great big bins awaiting the orders that never came. Some may have been new ammunition loaded from leftover Olin-produced components. Still more may have production run startup and stop overruns.
In December, Alliant completed acquisition of Blout's Sporting Equipment Group, including Federal Cartridge. Now Alliant was both operating the plant and selling XM193.
Then the Army steps in and halts deliveries. The word is that it is over liability issues.
All the headstamps on the Federal XM193 ammunition mean is that the cases are stamped LC01 and were intended for use in 2001 ammunition production. Cases intended for 2001 production can be manufactured in prior years and stored. You should not consider that the XM193 headstamps have any meaning at all other than to identify the cases as having originally been intended for production of 2001 Lake City ammunition.
Opinion: Some lots of 1999 Winchester Q3131 (but not Q3131A) had quality control problems. As a result many AR15.com members avoid Q3131 entirely and prefer Q3131A.
Opinion: Some lots of LC'00/LC'01 aren't properly sealed. Currently it appears that Lot 7 and some of Lot 6 is not properly sealed around the case necking.