While semi-automatics are useful, they suffer the same problem as all projectiles: ammo. Where's it all come from? How do you hold it? Reloading? One has to wonder what happened to those who left all the ammo lying around.
As for chainsaws, where are you going to store extra gasoline?
Bladed weapons seem to be better for long term. Sure, you lose the range, but you don't have to reload. Prefferably something light like a machete or katana.
I think the best thing would be a semi-automatic with extra ammo(cargo pants and ammo belts?) but not enough to slow you down too much. A few Molotov cocktails would serve well as well. However, one must have a light bladed weapon on the back.