Stephenson:Neal:Quicksilver:161:Blunderbuss (Professor Bikey Bike)

From the Quicksilver Metaweb.

A blunderbuss is an ancient and primitive sort of pistol. The short-barreled, large bore flintlock gun derives its name from the old Dutch words donder (thunder) and bus (gun). The widely flaring muzzle was designed to scatter shot at close range, making it a weapon of choice for home defense, as well as buccaneering and pirating. The British Sea Service (1790-1815) issued them as standard equipment since they could be shoulder fired or rail-mounted as a swivel gun.

Quotations

Some may well ask what a blunderbuss was. Indeed, this very question, it is said, was put to the Four Wise Clerks of Oxenford, and after thought they replied: `A blunderbuss is a short gun with a large bore firing many balls or slugs, and capable of doing execution within a limited range without exact aim. (Now superseded in civilised countries by other firearms.)' J.R.R. Tolkein --Farmer Giles of Ham (Tolkein is here quoting the Oxford English Dictionary.)

References

Startled to see this definition, and I have to say that I think gentlemen of the 17th and 18th Centuries would be as well.

There is certainly such a thing as a "blunderbuss pistol" (at least to modern collectors), but the word is typically used to refer to a short shoulder-arm, not a one-handed firearm.

As for the flaring muzzle being designed to scatter shot; perhaps... but any short barrel could accomplish that well enough. What the funnel-like belled muzzle did, that no other design could, was facilitate rapid reloading (which was, of course, then done from the muzzle end) on a bucking coach-seat or rolling deck. Getting loose powder and shot in something like measured amounts into a narrow orifice and tamped into place was difficult enough even on steady ground in stressful situations. Doing so in unmeasured amounts could constitute a greater threat than the intended target.