The problem is that there is barely enough other ammo as it is so it's best to try to setup bombs in strategic positions and take out multiple targets but doing this can be tricky because space is very tight and it's easy to rack up damage trying to set things up.
There is PLENTY of ammo. Once you get that first Flamethrower then you're set. After that point you get
a second Sparkshot
a 2 shot Rocket Launcher
a second submachine gun
a upgraded shotgun
a second flamethrower
a third submachine gun
a third sparkshot
the only fuel canisters that matter are the ones that show up before you get the Flamethrower from Zombie 41.
Zombie 10 canister - use it to kill zombies 11, 12, 13, and 14. Easy to set up, make the zombie drop it in the front of the store between the two shelves, run down the center aisle and fire.
Zombie 16 canister - kill zombie 15 with the handgun, make zombie 16 drop his canister near the same spot as before, try to kill Zombies 17, 18, 19, and 20.
Zombie 31 canister - press against the door to cause him to bust in lunging to get him positioned near front of the store again, sparkshot him to make him drop the canister, wait until most of the zombies get really close, use your flashbang grenade.
If you are lucky this explosion can kill zombies 32 through 38, but most likely will kill 32 to 37.
Zombie 39 canister. Just let the pale head zombie walk right near it and blow them both up.
Now you have your first flamethrower and things start to snowball and its just a matter of tapping the flamethrower instead of holding and letting the Fire damage over time effect work for you. Use Sparkshots on the pale heads and armored zombies with helmets, aim the rocket launcher at your feet if you get swarmed to AOE everything around you, use the shot gun to headshot other zombies, and SET EVERYTHING ON FIRE and the submachine gun on normal/poisoned zombies or dump most of it into a pale head to kill them.
I beat No Way Out, with Gunslinger and I can't beat any of the others on Normal. Hell it took me 3 tries to beat No Time to Mourn, with Infinite Ammo.