Full Sabrina is used with handguns. The problem is that it generally points the muzzle either directly up your left nostril or an inch from your ear. An AD would REALLY hurt. With a rifle it's not so bad unless you pull an Al Gore or use your shotgun muzzle to push your hat brim up (I've seen it done.) Again, bad idea.
I can think of times when pointing down is unsafe, but generally, when you're in the open on the ground, up is much more dangerous.