GIven almost every NCR trooper has semiautomatic AR-15's and the Legion chose to attack the Dam through the intake towers they've sealed their fate of defeat. That fact is backed up by the fact that in every ending (except the one were the courier works for the Legion) Oliver ends up bursting into Legate's main base of operations arriving victorious from the previous battle. That happens even if you destroy House's robot army, kill the Boomers and never help the NCR. That should tell you that Legate's plans still fail.

Why does that happen? Because the intake towers are a horrible place to attack from. All the NCR has to do is stand at the entrances of the intake towers and blast away. And unlike what happens when you play the game, yes one random NCR trooper from Boneyard can kill a Centurion in one shot. That's why tactically the Legion would still lose because those intake towers make the Legion easy pickings.

Besides the fact that the game endings back an NCR victory at the Dam, it's also backed by the repeated references that Oliver is amassing troops at the Dam. Which means in simple terms that every available troop that the NCR political system was willing to send is at the Dam, meaning that the Legion has to go against a [censored] ton of NCR troops who have fortified their position for years and yes have semi automatic weapons in mass, something that can't be said for the Legions basic troops who sometimes don't have guns.