Well, I hate to even say that a person that discharges any projectile into their leg is lucky but, this guy was actually pretty doggone lucky.
If the entry angle had been different AKS More Acute - he would have shattered his entire knee joint into splintery bite size little pieces.
"Mistakes happen, and if you practice stuff like this it is bound to happen." <~~~ That is just not a true or reality based statement at all.
This just did not have to happen though I'm sorry that it did.