I'd have to agree with the above advice. Shotguns are very efficient against small-fry all the way through the game. It also depends on what weapon you are using as your main. Plasma rifles are very ammo hungry and not very accurate (depending on traits) but dish out a lot of hurt. Assault rifles and hyperblasters (yeah, I've yet to progress to 0.9.9.7 yet) are relatively efficient but no good to you without WK. Even chainguns will carry a lot more killyness per ammo slot with the right traits (EE, SOB/TH) and mods, which you'll more than likely have. Always have one on switch, simply because it's so easy to find ammo for them. A couple of slots gives you 200-280 shots of buffer zone and pleb killing which makes things heaps easier on you.
You've got plenty of options really. A shotgun or chaingun for the smaller enemies. Then whip out your decked out plasma for the big guys.
If you still have problems, then for god's sake, do the Wall. That backpack will make any build incredibly ammo efficient, and bulking the ML makes for a good all-round weapon. You'll be switching between that and a chaingun as your secondary a lot, but it works.
Yes, MEn doesn't really offer a lot, but it's not too bad with the right tactics.