OK so here is what i have I used a Bushmaster Ar-15 the ammo tested was US military surplus(2 rounds green tips on right) and some UK military surplus (1 round unknown manufacturer) the rounds were fed into the magazine and each one was loaded into the weapon from the open bolt position. Here are the results: this is after 10 open bolt slams. As you can see the face of the primer is definitely dimpled. However the dimple appears to me no deeper than 1/32 of an inch. I don't have any fired brass from my weapon, and i didn't want to shoot anything off in my backyard so i don't have anything to compare against. After the first slam, the was a slight dimple, barley noticeable. After 2-3 more the dimple rounded out and became a little larger in diameter, however it did not get any deeper. So my conclusion is, a slam fire from a M-16 type weapon would be an extremely rare occurrence with a shell that was never loaded into the weapon. Even after 10 loads with the same shell, no slam fire occurred, so i believe this to be a unfounded rumor, or a freak occurrence.