I found a nice little present from the developers for this one, especially if you're loading this one up to do again, and find yourself without a good weapon.
Just past the gate that you have to power up to pass, the rail switch is set to steer you down the left path. However, if you get out and flip the switch to go right (through the wooden barricade), you'll find an abandoned car (and a karma point). On the right side of the car is a Kalash with a high-powered scope. There are some more military-grade rounds on both the right and left. Better, if you need to reload, you'll start back here.
The Kalash has a faster reload than a shotgun and the bastard, and you don't have to pump it. I got this the first time I tried with the gun.
Get back on Regina and go down the left track. There are roughly 4 waves of Watchmen, between 3-5 each.
If you like, you can stop after the second wave when you see the bodies on the right (as you face toward the rear) and go into the door for a karma point and some creepiness. The Watchmen will not attack the vehicle, and the game gives you a new save point here as well.
In all, I spent 113 rounds on this, mostly because I'm a poor shot. On the other hand (and please note that I didn't stop halfway through at the save point when I did my run), if you pop the achievement you can go back to where you found the Kalash and use regular rounds without having to worry about the Watchmen catching you.
When you get all the way to the end and Regina stops, your achievement will pop. If it doesn't, you can reload your checkpoint and try again.