Looks completely normal to me. It'll likely do it from the magazine as well, albeit not quite as deeply. The AR-15 has a floating firing pin, no rebound spring, so when the bolt slams home, the only thing to stop the pin is the primer. When you load from the magazine, it makes the same contact, but the bolt velocity isn't quite as high, so the pin doesn't pick up quite as much inertia. You could go to a titanium firing pin, reducing weight and decreasing inertia, or you could load from the magazine, or you could ride the bolt forward a bit before releasing it, or you could do like the rest of us millions of AR owners and ignore the "kisses" on the primers - you should be loading with the muzzle pointed in a safe direction anyway.