If I hold a pistol sideways and fire it, in the approved Hollywood fashion, in which direction will it recoil - horizontally or vertically?
Horizontally.
The barrel wants to move backwards, your hand acts as a fulcrum.
It’s independant from gravity.
The same reason bullets never fall to the ground.
The horizontal “gangsta” presentation is much maligned by firearms professionals. The pistol, if held in a conventional grip, will tend to kick upwards and backwards - which leads to the current preference for the two-handed bracing grip. This bracing against the kick is much more difficult to achieve if the gun is held horizontally, meaning that the shot will be less accurate.
But it looks way kewl 
It can also lead to nasty marks when the weapon hurts you. I’ll only speculate what my Taurus .44 or Smith 10mm would do if held in that fashion. This homie won’t go there.
I’ve shot my Glock 22 (.40) many times in this fashion at the range, and on the streets (j/k 
), without any ill effects. Surprisingly, I can get pretty close to my target. I think to do it accurately, you gotta shoot it fast, and pretend to be a real OG and shout obscenities at the target. 
I’ve also shot “gangsta style” a few times. It seems to recoil mainly horizontally, but also a bit vertically. I’d guess somewhere between 10 and 11 o’clock, for a right-handed shooter. You can shoot reasonably accurately, but you have to just point and shoot, if you try to use the sights you are gonna be nowhere the target
Now that’s an answer I like: if I am shooting something vertical, like an ostrich, I shall adopt a conventional grip; if horizontal, like a crocodile, I shall go gangsta. And if I am to pot something at a 45 degree angle, like the proverbial drunk using a lamp post for support rather than illumination, gangsta it is.
You’re gonna need some new sights…
Glad to be of assistance for ostrich hunting advice
“I always thought it was just some cold-blooded shit to say to an alligator before I busted a cap in his ass.”
In addition to the rearward thrust, pistol recoil also has a rotational component due to the rotational acceleration of the bullet by the rifling. In the normal shooting position this produces a slight rightward recoil, in the “gangstah hold”, upward.
In addition to the rearward thrust, pistol recoil also has a rotational component due to the rotational acceleration of the bullet by the rifling. In the normal shooting position this produces a slight rightward recoil, in the gangstah hold, upward.
That is true only for the glock in your right hand. True ganstah style is with a glock in each hand.
Of course in the southern hemisphere, the right hand gangsta hold results in a recoil that’s rotates slightly downward.
What if the gangsta is on a treadmill?
In the two-handed gangsta mode, the Glocks (Glockstas?) will throw empty cartridges in opposite directions. The right one will throw them up and to the left, and the left one will throw its brass down and to the right. There is no predicting exactly where the brass will go. In a standing position, though, the left-hand gun will eventually throw a hot shell in your shoe. This will be distracting and mighty darned embarrassing, especially when the thug you’re shooting at begins to laugh at your sudden gangsta dancing.
EXCELLENT!! Thanks for that one, CG…
I solve that problem by C-walking whilst I’m ruining muthafuckas like it ain’t no thang.
If he’s in an airplane on a treadmill, his quack won’t echo, and no one knows why.