The term ‘Blunderbuss’ originates from the Dutch word donderbus which is a combination of donder meaning “thunder”, and buss meaning “pipe”.
The transition from bunder to blunder was thought to have been deliberate after a few mishaps and lots of loud noise!!!
The earliest versions of these weapon were German Wheellocks from around 1590, and spread later to the Low Countries where Henrick Thielmans of Echten took out a patent on 26th October 1598.
Other countries became interested and they were manufactured in Flintlock and Percussion. Most were not very long and it is interesting to learn that the earlier ones were only about 18 inches overall (46cm)
A common misconception of these weapons is that they were loaded with anything including nails, broken glass, stones and gravel, in a dire emergency it could have happened, but it would ruin the bore.
Another misconception was that the wide muzzle made the spread of shot greater but it was actually the shortness of the barrel that spread the shot.
These weapons were loaded with powder, wad & lead pistol balls or swan shot and an over shot wad similar to a shotgun.
One of the reasons for the funnel shaped muzzle, apart from scaring the living daylights out of you if you happen to be looking into it, was to make for easier loading when riding on a bouncing stage coach or on rolling ships deck. The short length of the blunderbuss was handy for discouraging highwaymen on a prancing horse and much more convenient for use in crowded spaces, ie:- when boarding another ship.
The allowance of blunderbusses in HM Ships back in 1684 was one to every ten cannon carried, thus a ship with a hundred cannons would have ten blunderbusses.
The blunderbusses used by the Royal Mail coaches invariably were fitted with a spring loaded bayonet, and backed up with a pair of pistols too.
The pistols were known as “dragons” as they belched fire from their mouths, this term was later changed to “dragoons” as we know it today.

Above and left are pictures of my oldest flintlock Blunderbuss. It's a Royal Navy one with a two stage 15 ¾ inch (40cm) flared brass barrel, a banana shaped lockplate (shown above) marked “Tower 1746” typical heavy brass furniture, and a butterfly sling swivel on the side plate for carrying it aloft. (first time I’ve come across this refinement)

The next one (Shown below) is also a Royal Naval flintlock, these larger ones are sometimes referred to as Musketoons, with a 22¾ inch (58cm) steel barrel with a 2¼ inch (5.7cm) flared muzzle. Weighing around 6 kilos it would be a bit much to fire from the shoulder so it is fitted with a rowlock so it can be used as a swivel gun from the taffrail or bow of a boat. Once again typical heavy brass furniture the large Brown Bess type lock is marked “Crown GR and Tower” (c1780).

The next one (below top) I have is called a Flintlock Sea Service Musketoon. It has an 18½ inch (47cm) flared steel barrel, brass furniture, large lock marked “Lacy & Co and London” (c1800-1815), definitely military styled, and probably more likely to be used on Merchant ships.

The next flintlock (below bottom) has a 15inch (38cm) steel barrel but it isn’t flared, the calibre is .75 and looks to be a cut down version of a pattern 1776 Brown Bess musket, heavy military type brass furniture, Lock marked “Thomas” (1776-1790) and the frizzen spring is fitted with a roller. If a short barrel spreads shot then this one would work very well.

Lastly, a Flintlock Blunderbuss Pistol (below left), with a flared two stage, 5 inch (12.7cm) brass barrel marked “London” and a side lock with sliding safety marked “Meredith & Moxham” (c1812-1820), brass furniture, trigger guard engraved with stand of flags and pineapple finial.

Also, a brass Flintlock Blunderbuss Boxlock Pistol (below right), with 3½ inch (9cm) two stage flared brass barrel, body engraved each side with stand of flags and “London” and “Knubley” (c1750-1800).

By the middle of the 19th century, the blunderbuss was considered obsolete, although I have come across a few that were converted to Percussion lock, and I recall one made in percussion with a back action lock.

Kindly contributed by John Carter with references to "British Pistols & Guns 1640-1840" by Ian Glendenning and "Blunderbusses" by D.R.Baxter