Assault Trooper Achievement WILL NOT unlock

[Nylan]

I have every other single player achievement, including 'grizzled veteran', but I cannot unlock 'assault trooper'.
I am NOT using grenades (neither smoke nor frag) and I am NOT using melee atacks. I tried once w/ the bar and switched to mp40 when I ran out of ammo, then again, just with the mp40.
Now I will try with just the bar.
Any ideas?
Do you have to do it on a specific difficulty?
Its weird because I haven't had a problem with any other achievements.

EDIT: Finally got it to unlock, Bar only, didn't die, no melee, no grenades :)

[JesusBread]

MP-40 is not considered an assault rifle. That's where you went wrong;).

The Bar, MP-44 and the FG-42 is are the only assault rifles.

[Tained sainity]

ah didn't know that either, I would use one of the weapons that he has suggested and do it on the second level 'the island' it's short and their are lot of enemies and they will proberbly have the ammo for that gun and do it on easy take less to kill saving more ammo.