Leg locks are no good. In fact, in many Brazilian Jiu Jitsu (BJJ) dojos, you are not even allowed to think about leg locks until you are a purple belt. Leg locks are too dangerous. In MMA, you shouldn’t go for leg locks at all. You end up in bad positions when you go for leg locks. You put yourself in vulnerable positions, especially when you go for a straight leg lock (aka Achilles lock). Your opponents will be in good position to throw chingasos.
Do you believe any of that? Then when do you learn leg locks if you start taking Sambo? Exactly! If you believe that leg locks are too dangerous for MMA, you should have a talk with Shinya Aoki. In Dream 15, Aoki took on Tatsuya Kawajiri for the Dream lightweight championship. Within a few seconds, Aoki shot in on Kawajiri and grabbed a hold of his leg with a firm lock. Both men rolled around for a while. Take a look at the video to see how the fight ended.
Here is a good instructional video on how to finish a straight leg lock. They do a good job of explaining the details that will make the difference between you getting the submission or you receiving the chingasos.