At 10 feet number 6 shot is absolutely going to leave a mark! It may not stop a determined attacker though.
As has already been said, without adequate penetration there is little chance of incapacitating an attacker. Incapacitation is achieved by loss of blood, or catstrophic damage to the skeletal system or the central nervous system. That is the only guaranteed way of stopping a committed bad guy. There are volumes of research studies done by military, law enforcement, academics, and curious enthusiasts worldwide to support this. The tests have been done and many years of physical evidence all prove this central truth.
Adequate penetration cannot reliably be achieved with birdshot. That is a fact. What you believe, and what you load in your shotgun is up to you.