Well... This is one hell of a complicated topic will all the weapon combinations, skill and how players use them.
But frankly, the blunderbuss is mechanically broken in the sense of how it interacts with other weapons and the game itself. I'm going to break it down to three parts, first will be how the blunderbuss works in game, second will be it's interactions in fights and the third will be what we can do better.
What it does:
The blunderbuss is a flat out defensive tool, working the best on ladders and small ships like the brig's and sloops, it does it's job pretty well, killing people in one shot but... What does one do if someone just sits on your ladder? Take a shot you know won't kill them? Go get a different gun and find they climbed up while you where changing? Double gun but loose out on the sword?
The Blunderbuss just excels at close quarters engagements and nothing else, making it a stupid strong counter to the sword.
Combat:
The Blunderbusses main plan is to get in close, weather it's by running at the target, or waiting for the target to get close, you want to be point blank if your holding a blunder, and the problem is, two of the weapons counter this hard and the sword is the only one weak to this.
Making the blunderbuss in theory, a bad weapon.
What we can change:
The Blunderbusses roll is good, the only weapon that has a true defensive roll, and frankly I'd love for it to keep its roll without nerfing it to the ground. Here are the stats for the old and proposed blunderbuss.
Old blunderbuss:
Damage: 10 (X 10pellets)
Effective Range: point blank
New blunderbuss:
-Damage: 5 (X 12pellets)
+(more pellets)
+Effective Range: 0-7 feet (tighter spread, more reliable damage)
+Knock back based on distance to target.
+Increased knock back based on number of pellets hit.
+Shooting at the ground can launch the user flying. (Blunderbuss-jump)
So, with these changes, the new Blunderbuss works like this, the tighter spread gives the new blunder more range and reliable damage, can deal up to 60 damage per shot and deals a great amount of knock back, pushing players off ladders or cannons and giving the new blunder the bonus of a new form of vertical movement. Letting players jump high into the air or avoid possible fall damage by shooting the ground last second.
If a sword player rushes you with the new blunder, you can pushes them away then use your sword or secondary gun to finish them.
Overall I think the new Blunderbuss is better than the old one, instead of punishing sword users, it gives them a chance to deal damage and survive, making the Blunderbuss a spacing tool.
Agenst snipers and pistols you can use the new Blunderbuss to "blunder-jump" to them, closing distance through the air!
Allowing the blunderbuss to be a great defensive and movement tool, from pushing players away from your ladders to using the blunder-jump to board enemy ships that are out of reach to sword lunges.
