B&T can do a lot of fantasy genre games with very little hacking. To prove it, I'm going to give you all a very small hack of the game that converts it into a western fantasy type game in the mode of Spellslingers, Deadlands, Weird West or Owl Hoot Trail. Not much changes, but some things do.
There are a few changes with classes, as well as some name changes. Here they are
Assassin - Now called the Outlaw. No change
Barbarian - Now called the Brave. No change
Bard - Now called the Drifter. No mechanical change, but instead of songs, the drifter uses elaborate stage magic, manipulations of fate and stupidly long winding stories to perform his spells.
Cleric - Now called the Priest. No change
Druid - Now called the Shaman. No change
Duelist - Now called the Gunman. The Gunman can take a light pistol as a dueling weapon
Fighter - Now called the Brawler. No change
Monk - Still called the Monk. As you will see later, armor is not too important in this setting. To remedy this so the Monk doesn't lose their edge, I'd change their AC bonus to be forcing an enemy to make a re-roll on an attack against them once per combat, with an additional use every 5 levels.
Magic User - Not needed. Book learnin' is for sissies
Paladin - Now called the Sheriff. No change
Ranger - Now called the Scout. No change
Sorcerer - Now called the Mage. No change
Thief - Now called the Bandit. No change
Also, if you have the NOD Companion, I'd recommend throwing in the Psychic as the Mentalist and the Scientist as the Gadgeteer.
In a world full of guns with 19th century technology, plate mail won't do much against a .44 slug. Thus, every class gets an armor class bonus that increases as they level up. All classes start with a +1 bonus at level 1. This increases every other level for the Brave, Gunman, Brawler, Monk and Sheriff; every 3 levels for the Scout, Shaman, Priest, Outlaw and Bandit; and every 4 levels for the Mage, Mentalist, Gadgeteer and Drifter. The "good" ones should have a maximum of +10, followed by +8 and +6.
There are a few new types of weapons, and some characters become automatically proficient in them. If not, they can still be taken later with the appropriate feat.
Outlaws, Drifters, Gunman, Brawlers, Sheriffs, Gadgeteers and Bandits gain proficiency with all pistols.
Scouts and Brawlers gain proficiency with the rifle.
Brawlers gain proficiency with shotguns.
Archaic Weapons and Armor
The most common melee weapons in the current time period are daggers, hammers, picks (light and heavy), handaxes, saps, whips, throwing knives and muscle-powered bows. Armor is generally out of style and function. Though you can buy any piece of weapon or armor in the Player's Tome, it costs double the normal price. Further, non-magical armor is only half as effective against bullets (thus, plate mail would only provide a +4 bonus against a pistol)
Holdout Pistol - 100 gp, 1d8, 25/200, 1 lb
Light Pistol - 150 gp, 1d10, 30/250, 3 lbs
Heavy Pistol - 200 gp, 1d12, 40/300, 4 lb
Rifle - 250 gp, 1d12, 70/500, 10 lbs
Shotgun - 250 gp, 4d4, 10/40, 10 lbs
For every 10 feet beyond the initial range a shotgun fires, reduce the damage by a dice.
Handgun ammo costs 10 gp for 20, and longarm ammo costs 25 gp for 20
Cigarettes (20) - 1 gp
Good Cigar - 1 gp
Excellent Cigar - 10 gp
Normal Dice - 2 gp
Loaded Dice - 25 gp
Holster - 5 gp
Playing Cards - 5 gp
Marked Cards - 25 gp
Pocket Watch - 75 gp
Booze - 5 gp
Drifter Boots - 5 gp, 1 lb
Duster - 25 gp, 5 lb
Hat (Bowler/Gambler) - 10 gp
Hat (Drifter) - 5 gp
Covered Wagon - 50 gp
Train Ticket - 25 gp
Riverboat ticket - 15 gp
Wagon Ticket (200 miles) - 10 gp
Dynamite Stick - 25 gp, 2d6 damage in a five-foot radius
Gunpowder keg - 200 gp, 10d6 to a 20ft radius
Fuse - 5 gp, can delay a blast up to 10 rounds
Covering Fire and Pinning Down
In combat, you can choose to cover your friend's ass or scare the hell out of a target.
As an action, you can cover a friend for their round (as long as you have initiative before them). They gain an additional +1 to their AC for every person covering them. Covering fire costs 5 ammo
You can also pin down an enemy behind cover. If an enemy is getting pinned, they have a -1 modifier to hit for every character pinning them. Further, if they move out of cover, you can elect to take a free shot on them outside of your turn. Pinning an enemy costs 5 ammo
Any ranged magical enchantment can apply to a firearm just as easily as it can to bows.