One a day, plus iron

What to do when you hear gunfire (on the street), in relative order of importance:

1. Be willing to believe it is gunfire. Some people think it is firecrackers or backfiring. They sort of obliviously look around and wonder where the excitement is. If you hear multiple pops and it isn't the 4th of July or some other firecracker time, proceed to number 2.
2. Hit the deck. This lowers your target profile. Unless you are obviously the target, you are most likely merely in the wrong place at the wrong time.
3. Try to figure out what is going on.
4. Find cover.
5. Run/crawl away to put distance between you and shooter. See 3. Cover may be better than distance, depending on the circumstances.
6. Don't think that interior walls will help. Look for brick or something substantive. Cars, big trees, planters, even mailboxes are better than nothing.

Probably not a perfect list, but you can probably Google up a few other ideas from survivalist or pro-gun sites. Shooting back isn't really a good idea in most cases, though.