12 seems like too much when you think about the numbers. I'll just use Halo 3 for my example, since we have the most information about how its utility weapon works. The Halo 3 BR is almost perfectly accurate on the first bullet of its burst(the max deviation is 0.15 degrees from true center)
[1]. The Battle Rifle is at best a 4 shot kill, or in other terms, a 12 projectile kill with the headshot. If we were to assume that every player on one team focused on a single player from the other team, then the lone player would die instantaneously. Surprisingly enough, this isn't as bad as Halo Reach or Halo 4, where only a portion of the team was needed for an instant kill with a spawn weapon.
[1] Additionally, each bullet has a different margin of error, with the first round from the Battle Rifle being the most accurate of the three bullets. The first bullet can have an error between 0 and .15 degrees off of the true aiming vector. The third bullet is between 0 and 0.38 degrees, the second bullet falls somewhere between the first and third. It is a mistake to look at where the third bullet lands and assume all three bullets are that inaccurate. The outliers (0.15 on bullet 1 and 0.38 on bullet 3) are the absolute worst-case scenarios. In that context those numbers dont mean a whole lot but in the grand scheme thing of how the Battle Rifle operates, they are pretty meaningful.
http://halo.bungie.net/news/content.aspx?cid=14347
