All Marines qualify annually with a M-16 (MAX 500 yards).
The designated marksman uses a modified M-14 with a special made round.
If indeed the designated shooter was armed with a M 14 system, with the army 172 grain boat tails, the 400 yds is even less than what the rifle is sighted in at, which is usually 600 yds for the marine sniper teams.
I am damn happy that he can reach out and touch someone, and does so routinely and professionally.
My point is that for him and those trained like him, it is average.
I think if it is a "Corps" sniper rifle ALL of the ammo for that weapon is hand loaded at CL by the guy that last qualified that particulr weapon.