I've run through the game three times, so I have some
really good weaponry- you get to carry stuff over and new purchases of serious hardware open up. Also, Leon and Ashley get new outfits- Ashley looks like a Vegas slut, circa 1977. Right now I'm doing a mini-game in which Ada has to get five samples of the plagas, and there is no save game in this one. I can't get past machine-gun guy, and it's pissing me off.
(Since it's an Ada run I can't use Leon's stuff).
With regards to ammo, try not to re-load prior to upgrades- when you bump up your weapons capacity, it gives you full ammo. Have you tried setting back and using your rifle? It's nice because it only takes one shot, and if you have the villagers funneling through a doorway or tunnel you can take out two or more with a single shot.