When you try a metal cock ring make sure it’s not too small! When you put it on your penis is soft, fitting easily. But once you get a hard on and it proves to be too small, you have a real problem, risking damage to your dick.
It’s safer to use an adjustable ring (with snaps or a buckle) or a stretchable one, which are easy to remove.
Never leave a tight cock ring on for more than 20 minutes at a time. Restricting blood flow too long may damage the nerves and tissue of your penis.