Basic Trope: A wedding that one party is forced into.
Straight: Charlie has been carrying on a secret affair with Bella Sue. Her parents find out and force him to take her hand in marriage, whether he likes it or not.
Charlie is visiting a rural town and meets Bella Sue, who's instantly charmed by his city boy ways. They have a one-night stand. The next morning, Bella's overprotective daddy catches him and threatens to kill him unless he marries his dear daughter right now.
Charlie must marry Bella Sue because he had one conversation with her.
Downplayed: Two characters were already engaged, but an unexpected pregnancy prompts their parents to force them to Altar the Speed.
Justified: Charlie is a Jerkass who fully intended to take advantage of Bella Sue's crush on him and take off, not caring about any potential consequences she might face afterwards.
Bella Sue is perfectly at ease with the affair, but Charlie has an attack of conscience and tries to haul her to the altar over the objection of both her and her family.
Charlie could also blackmail Bella Sue's parents into letting their daughter be married to him if he decides to marry her. (In effect, it's the groom who holds the shotgun to the parents!)
Charlie and Bella Sue are forced to get a divorce at gunpoint by Bella Sue's daddy, who "never did like them city folk."
The parents force Bella Sue to marry Charlie if she objects.
Charlie's father is the one who forces him to marry Bella Sue at gunpoint, as a way of making his son take responsibility for his actions.
Bella Sue jokingly promises to marry Charlie. This prompts Charlie's parents to drag Bella Sue to the altar and stick the shotgun in her back. They don't trust those country folk to live up to their promises.
After sleeping with Bella Sue, Charlie meets her father... who sternly lectures him on his behavior and expects him to do right by his daughter without threatening him with a shotgun if they don't get married right away.
Charlie laughs, disarms him, then beats him up in front of his screaming and crying daughter.
Bella Sue doesn't share her father's values and finds the situation kinda ridiculous, so she derails the whole thing by refusing to marry Charlie. After all, she knows damn well daddy's not gonna shoot her over it.
The Priest doesn't go through with it.
However, Charlie blows off his advice, and Daddy goes to get his gun and forces the issue.
Charlie asks, Bella Sue initially refuses, then marries Charlie later when it feels right.
And then the priest gets a shotgun pointed at him for his trouble.
While driving through a rural area, Charlie notices Bella Sue standing by the roadside and cat-calls her. Daddy immediately shows up and shoves a gun barrel in his face.
Two shotguns are married by a NRA priest. Among the invites are several handguns and AK-47s.
Charlie knocks-up Bella Sue and is expecting this trope but he's a Lawyer and Bella Sue's Father says that he'd rather have a bastard in the family than a lawyer.
Charlie and Bella Sue say they're having a shotgun wedding... because they met over mutual love of guns, yet everybody else is surprised that Charlie is so happy.
Zig Zagged: Bella Sue's dad doesn't care about whether Bella Sue has an affair with Charlie, and things seem to go OK for them. Then her older brother Sam shows up and, catching them in the act, cocks his shotgun and threatens to kill Charlie. But then Sam sees that it's Bella Sue Charlie's having sex with, and says, "Oh, it's just Bella Sue. Carry on. I thought you were having sex with my other sister, Annie Sue." Etc.
City Slicker Charlie is visiting/stuck in the countryside. One of the following three situations:
Charlie doesn't even look at Bella Sue, the local girl, at all.
Charlie wants to have an affair with Bella Sue, but she doesn't go for it. (This might lead him to a more romantic attachment, but that's another trope).
Bella Sue's father doesn't give a damn what Bella Sue does with Charlie.
Bella Sue's father is angry, but he instead forces Charlie to leave immediately.
Bella Sue's father is genuinely pleased to meet Charlie, and offers him some money to take her on a date which he'd been doing in private already.
Enforced: "This is a story about a City Slicker in the rural Deep South. Audiences expect that he fools around with a local girl and be forced to marry her at gunpoint."
Invoked: Charlie and Bella Sue want to get married, but Bella Sue's father doesn't approve of Charlie and tries to pull a Parental Marriage Veto. Bella Sue then claims Charlie got her pregnant, figuring that her socially conservative father would rather let her marry Charlie than let her be an unwed mother.
Charlie judges the situation based on Daddy's family values and:
1. Buys his own gun. 2. Plans a detailed escape.
Bella Sue wants a nice, normal wedding, with a groom who's there because he wants to be, and asks her father to back down.
Charlie does the smart thing and keeps it in his pants, thus avoiding the issue entirely.
Discussed: "It's always interesting to see what these young men come up with for wedding vows at gunpoint."
Daddy's distrust and distaste of Charlie doesn't end after the wedding. ...Perhaps rightly so, since Charlie isn't exactly content with his new bride; if he doesn't manage to get a divorce or have the marriage annulled, he takes out his frustration on Bella Sue.
Charlie agrees to the marriage surprisingly easily, even offering to do it by the end of the week... then during the wedding, when everyone turns up, he and some thugs machine gun everyone in attendance. Turns out knocking Bella Sue up and getting forced into a Shotgun Wedding was a Batman Gambit designed to get her grandfather out of hiding since someone paid this Psycho for Hire to kill said grandpa—and the only way to lure him out of his hiding spot was his sentimentality over his granddaughter's wedding.
Charlie and Bella Sue are actually seeing each other on a casual but fairly regular basis, and are happy with that... up until Bella Sue's father marches them to the altar at gunpoint, at which point neither of them are ready to take such a big step, and the relationship starts to fall apart.
Charlie can't stand being married to Bella Sue and she isn't happy either. She had her eyes on Chuck Fin! Then they spend time together,find out they have more in common then they thought and make the most of it.