I would like to point out that Bart B. is correct that Lake City never got the accuracy from service rifles with ball powder.
Lake City was using WC846 for some M118SB lots, which has known delay issues with the the arsenal primers used by Lake City (this wasn't addressed until the mid 90's with the adoption of the m134 minigun that required tightly controlled ammunition to prevent out of battery firing). From a viced accuracy barrel, ignition delays aren't as visible in the resulting group as when it is fired by a human who is wiggling on target.
The other part of ignition where ball powders don't generally do as well is that they are normally used with magnum primers, which induces a larger pressure variability off the bat (based on German Salazar's testing). The larger the case, the less this matters, the smaller the case, the more this matters. This is one of the reasons that the 223 (while Palma legal) hasn't displaced the 308 for Palma shooting, when you are competing at that level of accuracy every bit of consistency counts.
Now the bad lots of M118SB would still shoot pretty tight, the Sniper School at Fort Benning considered a "bad lot" of M118SB to make a 5 inch group at 500 yards, right at MOA, and a "good lot" (loaded with IMR4895) would do half of that. But this is from the prone, with a supported M24 rifle. From a service rifle with just a sling you can see how it would really open up the groups with a bad lot.
Machine guns are awesome until you have to carry one.