I really liked these movies.
In the old "Dead Movies" the dead were slow and lumbering, thus you could use anything to shoot them- bolt action rifles, pump shotguns, etc. They were a minimal threat, unless a mob of them surrounded a house.
In "Dawn of the Dead" (2004 remake) (and in 28 Day later) the zombies obviously run like track stars and in that case you better have a semi-auto rifle with a big mag. The obvious challenge is getting good head shots.
As far as "Andy" the gun shop owner, I have to agree with Albanian. He should have tried to make a sizable dent in the zombie hordes with his sniper rifle. Sure, at one point it would get to be exhausting... but who knows, it might thin the herd. Do the math. How many 308 rounds does a gun store usually stock? How many zombies were in the parking lot? He couldn't have killed them all, but reducing the number may have helped with the escape later on. I'm not sure.
If you go back to "Day of the Dead" (1985) the best solution would have been to helicopter a commando team into a military based, secure arms plus an armored convoy... then come back for survivors and drive up the highway to DC or some other location with hatches buttoned up tight- laying down M60 fire when necessary.
Yes, it is easy to flame this topic as "pure fantasy" but it is fun to discuss.