I started urethral sounding a few weeks ago. I do it early in the morning on weekends. I bought a Hegar sound set that includes eight double-ended steel sounds. On each, one end is 1mm larger than the other. The sizes are as follows: 3-4mm, 5-6mm, 7-8mm, 9-10mm, 11-12mm, 13-14mm, 15-16mm, and 17-18mm. They are about 8 inches in length. I started slowly, but quickly worked my way up to 10mm which is the largest my dick can easily take, all 8 inches of it until it dissapeared into my shaft. The 11 mm on the other hand did not want to go in at all. I had to experiment with using two smaller sizes at the same time to slowly stretch the opening of my cock. You are not supposed to apply pressure with sounds but let the weight of them determine if they can enter and how deep. This morning, after a lot of stretching using two smaller sounds, I tried the 11mm again. I got my dick a little hard and did what you are not supposed to do, put a little pressure on it. Suddenly I felt a pop on the tip of my penis and it dropped in about half an inch, super tight, slight pain from the extreme stretching of the opening. Felt good though. I will be working with the 11mm for a long time. But I am happy to have broken that barrier. The other sizes were so easy.