Hi,
here is a test/introduction about a gas-alarm pistol, which is so close to optimum backup for selfdefence as possible.
The Czech-made ZVI Night Hawk IS the smallest (116 mm long) and lightest (400 gramms) 9 mm PAK semiauto gaspistol on the market.
(The Umarex Reck Goliath is longer and heavier!)
THIS GUN DO NOT HAVE PTB!!!
Technical Description:
The pistol is originally designed in 9 mm Browning Short (Kurz, .380 ACP) caliber as REAL pistol. The gas-alarm version is different only in barrel (a lot of barricades etc) and recoil spring.
Therefore the fit, finish and materials are excellent. The frame is aluminium alloy, every other part (except the plastic grips) is made of STEEL. 
The gun is simple blowback-operated semiauto with Double Action ONLY trigger, free-floating firing pin, NO external safety. Safe to carry "ready to go".
The magazine is 6 rounder (fstandard), single stack, or 8 rd (extended).
6+1 rounds of 9 mm PAK from such a small package is a great "firepower". 
Negatives: there is no slide catch, and no thread for pyrotechnic launcher.
In action:
We've tested 2 samples, the pistols were not cleaned, just get out of the box, load, and shoot.
6 rounds of Umarex CS and PV mixed: NO problem
2 magazines of RWS 9 mm PAK alarm: NO Problem
3 magazines of Fiocchi 9 mm PAK: NO problem.
Versions:
- basic black
- nickeled
- duo-tone
- black with wooden grips and golden operating levers and buttons
Summary:
The gun is modern-looking, made of good materials, seems to be reliable, great comfort when carry, and "bestof" in the world in two categories.
Yes, what is painful: you have to pay the price of the real pistol: cca. 300 euros. 
But it is in my pocket, and I love it:-)) 
(Photos by http://www.gunforum.hu by Szlejer and Scharm)