Updated: Aug 27, 2020
The so-called “thunder gun” is a large-bore long arm with a distinctive flared muzzle. It is not a hunting weapon, but a handy defense gun, a hard-hitting smoothbore “street howitzer” designed for close combat.
Its fearsome reputation and wide barrel gives it a +1 to Intimidation. The muzzle flare serves a purpose – it makes it easier to dump a handful of shot down the barrel, and provides a +2 to rolls when trying to reload in precarious positions (such as from a bouncing coach seat or a ship’s rigging).
Two different types of blunderbuss weapons are described below: the small, short, and lightweight “bedroom blunderbuss,” a weapon for repelling housebreakers, sometimes called a canoe or coach gun. It has a 14” steel barrel.
The larger one below is a heavier shipboard weapon sometimes called a “rail gun,” as it was fired from the ship's rail right before boarders leaped on board. It was also, confusingly called a musketoon -- not to be confused with a cavalryman's musket. It was a much lighter version of shipboard swivel gun artillery. It has a 25” brass barrel and a 2” bore and typically is loaded with grapeshot.
A rarer type of blunderbuss is the double-blunderbuss. This was equipped with two barrels, two locks, and two triggers; both barrels can be fired separately or together, without penalty. A single ramrod is fitted below the barrels. Some examples include a spring bayonet. It cannot be fitted with an oval muzzle (below).
Blunderbuss weapons are flexible—they can fire a single large solid shot, so-called pumpkin balls -- as well as shot of various sizes, such as birdshot -- but usually fired buckshot. Adventurers may be most interested in using buck-and-ball, which fires one full-caliber ball and a few buckshot pellets, or they may prefer a double or triple loading if facing a really dangerous adversary. A popular load is “grapeshot,” a handful of very heavy shot of approximately .40 caliber.
A blunderbuss can be fitted with various accessories. Since its flared muzzle is too large for a socket bayonet some are out-fitted with a spring bayonet, which is attached above the barrel and flips down and locks in place when a thumb-activated retainer is pushed on the tang of the stock (a Ready maneuver, or a Free Action with Lightning Fingers). It uses the Spear skill, Reach 1, thrust+1 impaling. It takes two Ready maneuvers to fold the bayonet back. +$50, +1 lb.
Some are fitted with an oval muzzle, rather than a flared one. This acts to spread the shot and change the shot pattern from a circle to an oval which will “lie” horizontally; it gives +1 to hit human-sized targets (SM 0), but also has effective Rcl 2; this means hitting is easier, but the number of hits is reduced.
Sawing off the shoulder stock of a blunderbuss was a favorite of pirates. It removes -1 from Bulk and lowers weight by 1 lb. It also reduces Acc by 1. Multiply ST by 1.2 (round to the nearest whole number) and increase Rcl by 1 (but not if Rcl is 1). A folding stock can also be used.
WPS of 0.12; CPS is $0.50.
Buckshot, dmg 1d pi, Acc 2, Range 30/600, Wt. 6/0.12, Shots 1x13, RoF 1(30), ST 11†, Bulk -4, Rcl 1, Cost $200, Notes [1, 2].
Grapeshot, dmg 1d+2 pi+, Rng 30/450, RoF 1x6, Notes 
Birdshot, dmg 1d(0.5) pi-, Rng 5/330, RoF1x22, Notes 
Ball, dmg 4d pi++, Rng 90/900, RoF 1, Rcl 6
Double Ball, dmg 3d+1 pi++, Rng 45/450, RoF 1x2, Rcl 1
Triple Ball, dmg 3d-1 pi++, Rng 30/300, RoF 1x3, Rcl 1
Buck and Ball, dmg 4d pi++, Rng 80/800, RoF 1x5, Rcl 1, Notes 
w/ dmg 1d pi, Rng 30/600
Double Blunderbuss, 4-bore.
This is the same as the blunderbuss, above, but with two barrels.
WPS of 0.12; CPS is $0.50.
Buckshot, dmg 1d pi, Acc 2, Range 30/600, Wt. 9/0.24, Shots 2x13, RoF 2(30), ST 11†, Bulk -4, Rcl 1, Cost $500, Notes [1, 2].
WPS of 0.2; CPS is $0.50.
Grapeshot, dmg 1d+2 pi++, Acc 2, Range 75/750, Wt. 14/0.2, Shots 1x13, RoF 1(30), ST 13M†, Bulk -5, Rcl 1, Cost $500, Notes [1, 2].
† The weapon requires two hands. If you have at least 1.5 times the listed ST (round up), you can use a weapon like this in one hand, but it becomes unready after you attack with it. If you have at least twice the listed ST, you can wield it one-handed with no readiness penalty.
 Sling swivels.
 The buckshot load provides +3 to hit due to RoF bonus; the heavy blunderbuss grapeshot gives +3 to hit.
 The grapeshot load provides a +1 to hit due to RoF bonus.
 The birdshot load provides a +4 to hit due to RoF bonus.
 Treat as a shotload, Rcl 1; the first hit scored is always with the full-caliber ball, while any further hits are with the buckshot. The load gives a +1 to hit due to RoF bonus