TEXT: Dueling House Rules for
  Snakes & Saloons
  Dueling has been a long standing tradition in many regions
  across the land. Duels were created in order to settle
  disagreements in a more civilized way than simply tearing at
  each other’s throats. Duels are often called by individuals
  who seek retribution for an act of dishonor conducted against
  themselves or someone that they represent. Duels are not
  conducted in order to slay the accused, rather they serve as a
  means for the accuser to regain their honor. However, if
  someone expires as a result of the duel, so be it. Some areas
  have outlawed dueling, seeing the practice as uncivilized and
  barbaric.
  Code Duello
  Duels are often conducted with opponents using matching
  weapons, however this is not always necessary. Individuals
  can chose to use whichever weapon they feel the most
  confident with, barring that their opponents agrees. Each
  opponent must also agree to the duel’s specific rules, called
  ‘code duello’. It is in the establishing of the code duello that
  opponents can raise any rules that they wish to. These rules
  are boundless, but some of the common rules that are
  established are: matching (or not matching) weapons, the
  allowance of post-duel medical attention, the allowance of
  magical weapons, equipment, or effects, the number of paces
  that will be taken, and any prizes associated with the winner.
  Once the code duello is established and both opponents
  agree to it, they shake hands and proceed to the dueling
  grounds. Code duello may also be established any amount of
  time before a duel is to take place. Any violators of the code
  duello are seen as both dishonorable and criminals.
  Punishment for a violation of code duello can be as intense
  as murder, depending on how much honor is valued in the
  prosecutor’s society.
  Conducting a Duel
  When opponents meet for a duel, they meet face-to-face and
  offer to have their weapons inspected by their opponents and
  anyone serving as a witness. If there are no problems in the
  weaponry, the opponents stand back-to-back. They take a
  number of paces agreed upon, making careful effort to match
  their opponent’s pace. When both feet settle on the last pace,
  opponents are permitted to turn, draw, and attack.
  If both opponents have gotten to this step without
  (noticeably) violating the code duello to their opponent or any
  witnesses, these special duelling rules take place:
  Both opponents make the following rolls (including
  modifiers) and total them:
  An initiative roll + an attack roll with their dueling weapon
  + a damage roll with their dueling weapon
   
  The opponent with the highest total score wins the duel. The
  loser is brought to 0 hit points, knocked unconscious, and
  immediately makes a death saving throw. At this point the
  duel is over and the loser may receive medical attention if the
  code duello allows it, otherwise the loser is left to their fate.
  Individuals may duel with melee weapons against
  opponents with ranged weapons, however they must be swift
  and should take careful consideration when establishing the
  number of paces taken in the code duello in order to make
  sure they can close the distance. Every pace is 5ft of distance
  between opponents. Opponents who are dueling with melee
  weapons and take a number of paces which places them
  beyond their movement speed must make both their initiative
  roll and attack roll with disadvantage. If the number of paces
  they take exceeds double their movement speed, they
  automatically lose. Opponents who are dueling with ranged
  weapons and take a number of paces which places them
  beyond their weapons first range must make their attack roll
  with disadvantage.
  If letting your Players or NPCs fall in one strike feels
  inconsistent with traditional combat in which these
  characters could normal take a few rounds and keep going,
  remember that when dueling opponents are making
  themselves purposely vulnerable for the tradition. This is
  emphasised by Armor Class not playing a key factor in
  duelling.
  Example Duel
  The gunslinger Winston Jackson calls a duel with the outlaw
  Maybee Sinclair to reclaim his sister after Maybee had
  kidnapped her. If Maybee agrees to the duel, they then
  proceed to establish the code duello, if not they settle things
  the old fashioned way.
  Winston demands that if he wins the duel, Maybee’s
  henchmen will release his sister back to him. Maybee accepts
  on the condition that if she wins, Winston will become her
  servant for the foreseeable future. Winston agrees and it is
  added to the code duello. Winston then states that he wants
  to take 5 paces which amounts to 25ft. Maybee argues for 6
  paces, 30ft, they agree and it is added to the code duello.
  Then Winston argues that there will be no post-duel medical
  attention. Maybee agrees on the condition that they must also
  use matching non-magical revolver. Winston accepts and it is
  added to the code duello. Winston and Maybee have nothing
  more they wish to add to the code duello, they shake hands
  and exit the saloon which Winston had confronted Maybee
  in.
  They step face-to-face onto the dirt road outside the saloon.
  They both produce revolvers and they inspect each other’s. A
  sage who’s been spectating the whole ordeal offers to ensure
  that both revolvers are non-magical. Both agree and the sage
  casts the Detect Magic spell and sees that neither are
  magical. The sage returns them, Winston and Maybee holster
  the weapons and stand back-to-back. They each take 6 paces,
  making a distance of 30ft between them. When the last step
  of the 6th pace is taken, both of them turn, draw, and fire.
  Winston’s dexterity modifier is +5 and his proficiency
  bonus is +3. Maybee’s dexterity modifier is also +5 but her
  proficiency bonus is +4. Both make initiative rolls, attack
  rolls, and damage rolls simultaneously.
  Winston will roll: (1d20 + 5) + (1d20 + 8) + (1d10 + 5)
  Maybee will roll: (1d20 + 5) + (1d20 + 9) + (1d10 + 5)
  While these opponents are at about the same dueling level,
  not all duelists will be and that plays an important factor into
  whether or not someone will accept a duel.
  Winston rolled: (21) + (27) + (8) = 56
  Maybee rolled: (13) + (26) + (12) = 51
  
  With a bit of luck, Winston’s rolls totaled higher and thus he
  was able to draw and fire faster and more accurately than
  Maybee. He won the duel. Maybee suffered a terminal shot
  and is knocked unconscious and immediately makes a death
  saving throw. Per the code duello, Maybee is not permitted
  any medical attention. Only the dice will tell her fate (unless
  someone violates the code duello and heals her, of course).
  Winston walks away knowing that, if all goes to plan, his
  sister will soon be returned to him. Optionally, Game Masters
  may allow the winner to kill the loser outright if it is
  dramatically or narratively appropriate (though we do not
  recommend you do this with Player Characters for obvious
  reasons).
  
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SOURCE: textdata/thevault/Dungeons & Dragons [multi]/5th Edition (5e)/3rd Party/Snakes & Saloons/Dueling House Rules for Snakes & Saloons.pdf