This whole "shotguns are useless" thing people need to stop thinking tacticool shotguns.
its not really the shotgun but the ammo.
how many rounds of 12ga ammo can you realistically carry while still being able run from zombies (remember they run too now)
i would go with everything in .22 and carry several thousand rounds of ammo in my pack - which you can scavenge anywhere.
a light,
accurate 18" barreled bolt-action .22 rifle with a compact scope, folding stock (and backup iron sights). you can fire
any .22 ammo (even shorts and target ammo) and its much more reliable than a semi - you can fire thousands of rounds from it without worry about cleaning or something breaking. since youll most likely be using this from safety like the roof of a shopping mall, you have the luxury of taking your time and taking headshots. you can carry the rifle and thousands of rounds of ammo in a backpack still be able to run around.
then for close work, something like this (also in .22)
[youtube]QS31Gm9tIig[/youtube]
^should still be accurate enough to pop their heads with single shots up to ~75 yards.
theres ~10 rounds of buckshot to a pound, so ~100 rounds of buckshot = 10lbs.
about 2.5
bricks (500rds each) of .22LR = 10lbs.
and a Katana? pfft... go medieval on the #$@%ers: