Modelled a 7.62x39 round in Onshape. All dimensions should be accurate (apart from the bullet itself, as it varies). If it does not print well, I suggest splitting the model in a program like NetFabb and then gluing the pieces together.

The 7.62x39 is one of the most widely used rounds in the world. It was developed by the Soviet Union and first officially used in 1944, and first saw use in the famous AK-47 and SKS rifles. It is still a popular round today due to its stopping power and availability.

Have fun!