I do think the accuracy for the units needs to improve, I had Marines shooting down at a stationary target around 230 meters away and it took more than 10 rounds with an M16a4 to hit him while they were prone. If I recall correctly the units were not exhausted and barely moved into position from a LAV, with a clear line of sight it should have taken 2-4 rounds at the maximum to hit the stationary target.
I can see a poor sightline at 400 meters taking a few hundred rounds, mix an oversized fire team element and I would put around a thousand rounds of small arms into a small building while moving closer to clear it unless you had a 50 cal with SLAP rounds you can probably use a box or two.
The article posted makes sense, training on ranges before deployments, ranges on deployment, and suppressing a target with sustained fire, training alone take a few hundred thousand rounds. The most brass casings I've ever seen was in 29 palms, fun times