Phriend, while double-barreled pistols certainly exist, they are not considered as practical for self-defense needs as a snubbie revolver or compact semi-auto pistol. Why? Well, both of the handgun designs I mentioned are multi-shot, something worth considering in a life-threatening situation. Also, some derringer designs are considered crap. Now, Bond Arms does produce a quality derringer with a barrel length over 4 inches. Unfortunately, the size of this gun approaches that of small revolvers and compact semi-autos, thereby negating its virtue of compactness.
Why do people still focus upon derringers? Mystique. History and Hollywood have popularized this small gun in the public imagination. So, the derringer remains an option for this reason. I hope my explanation helps.
If you are determined to purchase a derringer, then I advise buying one of good quality chambered for an adequate caliber. There is one more piece of advice I will also offer: practice. You will need skill to use it effectively in a stressful situation. This is the best information I can offer.
Timthinker