Guide
woohoo! completed without using the cheap cheat of esc when reloading :P
first off, try to maintain the barrier as well as possible. use 6 hrs to find other survivors, and another 6 to find new weapons. fix ur barriers only when its really low (like below 50%).
when u get ur first new weapon (revolver), equip it as your backup, since the slot is free anyway. imho the handgun is much more effective though.
when u haf 3 survivors and above, u can start getting ur barrier fixed without worries. dun overfix though, not even for 1%.
once u haf ur usp, equip it as ur main weapon. if u wish, u can use the shotgun as a backup to clear those pesky undeads that r too close for comfort. try not to exhaust the 8 bullets, cos once u start reloading it, u're most likely a goner (alternatively, try the esc trick, though it might screw up ur game, in which u've somehow cleared all the zombies on-screen, but the game juz doesn't end).
u will surely get other weapons as u proceed throughout the game, but stick with uzi and shotgun. 50rounds/clip of uzi makes reloading much shorter n avoids the situation where the undeads juz start piling up infront of u while u try to reload. its firepower might be slightly less than m14a or ak-47, but its high ammo more than makes up for it. try reloading as u move up and down along the barrier, instead of when ur ammo runs out.
using uzi, u should clear the levels with ease, until u manage to get the final weapon. beretta. at first i thought its the wimpy handgun, and i was like "wtf?!" but nono, its the ass-kicker m501 sniper! a measly 10rounds/clip? no problemo, its shot is so powerful it goes thru' them undead! like awp in cs :D. n yea, it's one-shot-one-kill, if u aim for the heads. the only exception are the swat n marine zombies; aim for their legs instead.
by the time u get the m501, the game shd more or less be ending soon (at level 20). enjoy the ending!