I have always heard that John Lennon meant it as a reference to Heroin.

Meows right, it was a drug reference. They knew they couldn't get the name of the drug passed the censors so they used gun instead.

Metaphor

I was always told that gun was an in word for heroin, but Wikipedia denies this. The original, working title of the song was "Happiness Is a Warm Gun in Your Hand", which was inspired by a magazine cover containing the phrase. In 1962, Charles Schulz wrote a small, square Peanuts book titled "Happiness is a Warm Puppy," which sparked the snowclone. Some people think that the song title refers to a needle for injecting heroin. Lennon denied this. According to Wikipedia, "Lennon once claimed the song was "sort of a history of rock and roll," as it features five different sections but is less than three minutes long. The song begins with a brief lilting section ("She's not a girl who misses much..."). Drums, bass and distorted guitar are introduced as this portion of the song proceeds. The surreal imagery from this section is allegedly taken from an acid trip that Lennon experienced. After this, the song transitions into a Lennon song fragment called "I Need a Fix," built around an ominous-sounding guitar riff. Lennon's plea "I need a fix/cause I'm going down" in this section forms the basis for speculation that the song is about heroin addiction (indeed, sources have claimed that Lennon was addicted to heroin at this time; Lennon's heroin addiction was referenced famously in "Cold Turkey"). The final section is a doo-wop send up, with the back-up of vocals of "bang, bang, shoot shoot". The song's multiple sections would inspire Radiohead's three part epic Paranoid Android on OK Computer." Why Lennon would use the reference is unknown, but he was into abstract lyrics as well as into shock at the time, so well....it was the 60s.

I thought it was a sexual referrance to digital penetration (or "fingering"), though I suppose it probably more likely refers to heroin. The only problem is heroin is cold so if you take that into consideration it really sounds more like it refers to the vagina.

The lyrics have many levels to them. First of all I think the warm gun is a reference to the penis. If you read the lyrics closely there are a lot of sexual references being made in the song. Also ... the lyrics for the song came from a magazine Lennon looked at that said this "Happiness Is A Warm Gun" - I think he took the lyric for how ridiculous it is. So no.. the song is not pro-gun... It's more a joke at the type of people who get off on guns. At least that's one level of the song. Like I mentioned, there are others.

I think it means Sex. Hence "Warm gun"- Penis. "Bang bang,shoot shoot" simply means the big finish in sex for mean when they cum.Most likely because people think sex is happiness.

Huntan is right. John saw that exact phrase on the cover of a gun and rifle magazine. He was often inspired by bits and pieces he picked up in books, magazines and on TV (such as "Good Morning, Good Morning", which he picked up from a Kellogs Cornflakes commercial). Then he goes on in the song with his typical "gobbledegook", similar to "I am the walrus". He also declared that the lyrics "she's not a girl who misses much" was a phrase his aunt Mimi often used to described a girl who was not "proper". The lyrics "mother superior jumped the gun" refer to Yoko Ono. In other words, the song is in part inspired by different sources and in part it is Lennon in his typical "nonsense" style. Many Beatles songs are said to be referring to drugs, but in reality, though several were probably "inspired" by drugs, not very many make direct reference to them. Strangely enough, some that do refer directly to drugs are not recognized as such. He was definitely NOT advocating violence with this song.

It is a drug reference about a heroin fix, the drug John was using at the time.

i always thought this was about infidelity and revenge? and that happiness is a warm gun was a direct translation off of an advert john had seen somewhere...

SExual innuendo Lennons tongue in cheek song