Bah, they can't "backfire"...there is only one way that nail can come out of the gun unless it explodes. That's like saying a second bullet came out of the back of the pistol.
I picture the following;
Like most guys he keeps the trigger pulled and used the safety shoe to fire the gun on contact. Those guns, especially the framing (4"nail) guns, are pretty heavy, so the easiest way to hold them when you're not shooting is to let your arm hang down or hold the gun upside down balancing on your hand. It's the same position you would hold a pistol to have it pointing straight up in the air.
Now two things;
One, the gun being heavy you'd give it a pretty good jerk to get it up to that position.
Two, it ends up being right next to your head.
I can easily picture him jerking the gun up and smacking himself in the mouth with it. It hurts like hell...it's embarrassing...and it might bleed a little.
Any shot to the head is a little disorienting and I can see where he might not know the gun went off when he smacked himself.
OR...he doesn't want to admit he was shot by a jealous husband.
