Full Auto

This achievement is a little bit glitched.
After a few tries and confirming it with other players i figured out how to unlock it.

Tutorials ----> Rampage and make sure only to get 5 in a rows. If you do more use the unwrack button to rewind.
You have to finish the race in time to unlock it.

Try this on one of the Rampage series races where there are a ton of cars that are available to destroy with a single missile shot.

You need to kill five cars close together. I'd estimate about 15 seconds apart should still count toward your chain.

Near the end of the race, once you have killed five in a row (you'll see text on-screen reading Two, Three, Four In A Row, then Five In A Row), let your chain die off. Don't kill anybody else before you finish the race.

This Achievement does not stack with the Rampage Achievement, nor will you earn the Three in a Row Achievement at the same time. You need to hit a maximum of five in your chain.

The "in-a-row" achievements in Full Auto are a bit wonky. They do NOT stack and have the weird property where you can only unlock the BEST achievement that you would have earned if you had none of the "in-a-row" achievements.

Example 1:

You have rampage, but not five-in-a-row or three-in-a-row.
You race and get six in a row.
You earn five-in-a-row.
Why: The "best" achievement is the one the game tries to unlock, which is five-in-a-row.

Example 2:

Same situation: You have rampage, but not five-in-a-row or three-in-a-row.
You race and get rampage (eight in a row).
You earn nothing
Why: The "best" achievement is the one the game tries to unlock, which is rampage. You already have it unlocked so it can't pop.

This is very strange, but I can confirm this is how it works (at least for five-in-a-row). It's probably because Full Auto was an early title and the developers were unclear as to how achievements worked.

The way to earn this is to do the rampage tutorial. You have to get at least 30 kills, so bump your kill streak to the max you want to unlock. When you hit that, wait for the kill roster to the left to disappear and give it another 5-10 seconds. Now it's safe to get more kills. Be careful you don't accidentally fire a rocket into a crowd and get a rampage and watch your time. You still have to finish the race successfully.