Doug38pr,
Do you lie awake at night thinking of these scenarios?
You are walking down the street and some nut comes out of a bank with a magnum handgun or a full semi auto rifle.
Is a "full semi auto rifle" something different than a semi-auto rifle? Or were you describing a full magazine?
Okay, to be serious...
Most commonly available cover will be behind an automobile's engine compartment, usually aligned with the tire. The engine provides cover against most ammo and the tire/rim against rounds fired low from taking your legs or ankles out.
Thick trees are usually excellent cover. Wood is dense and will usually slow or stop many FMJ rifle rounds if it's big enough to hide behind.
Concrete walls or K-rail barriers will also work although a rifle with AP ammo may penetrate, depending on distance and angle. Concrete "planter boxes" filled with dirt will work well and some can be found at the base of lamp posts in parking lots.
If nothing else, a telephone pole works until you can spot larger cover (though some of us full-figure types will have to suck it in to hide behind one).
If you're in a shopping mall, strip mall or other retail store, don't expect those solid looking pillars to offer much protection. They're often cosmetic shells for looks. Likewise for pillars at strip malls -- they're stucco covered plywood. Even those "granite pillars" at office buildings are just a thin tile placed over plywood. Likely it will stop a .357 but not a rifle's FMJ round.