Sorry for the double post, but I can't edit my last post.
These glitches should have be fix already. Granted, I don't know what's the problem with the code is but from my understanding a simple if statement should do the trick. You already keep track of weapons stats for each level on HaloWaypoint so.
if (Map = Shatter && Shater's Detached Machine Gun Turrent >= 20 total kills)
then Achievement unlocks
Honestly it's sad that the Xbox's flagship series has Achievement glitches.