When choosing the cut to make a good old lamb biryani it's good to bear in mind to add some bones too. Just count them on the way in to know how many you need to find at the end and take out. The addition of some bones it's terrific and how these add to its favor. I love bones personally and do not have a problem with finding these in my biryani but if you don't, simply fish them out at the end when serving. In terms of what parts of the lamb or mutton you could use, use any! I have used lamb shoulder, lamb leg, and even lamb neck fillets or the actual bony part. Here is more on lamb, how to choose the best lamb, how to store lamb meat, and other useful tips.