Ammochain build has its little trick to pull it off.
It is the best to consentrade on using shotguns till the point you find a plasmagun and only use the chaingun on bigger baddies.
Shotguns DO Benefit from SOAB a lot.
The thing is that to win in Nightmare you are going to have to be able to kick ass and survive. Otherwise you won't be able to get your levels.
The best way to win in Nightmare is to corner shoot and scout a lot to kill all the bastards before they see you. Ofcourse if you want a berserk or invulnerability, then you should go on a rampage.
Imps are one of the biggest problem of Nightmare as they are very un predictable.
Few more tips;
-Always check the vaults(not the special level) if you get a level feeling about them, even though they are usualy filled with cacodemons, the loot is more then worth the risk. I've many times found a plasmagun on level 2 due of vaults.
-Open door, shoot, close door, reload, open door, shoot..
-Abuse barrels. Lots of corpses? Acid barrel over them and fire, no more corpses.
-Imps first, then the hellknight.