Background info:

He's 14.
He's a Jack Russell.
He has always been temperamental.

I don't need "It's the owners fault, you should have trained him better blah blah blah" I don't need reasons, i need solutions.

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Week in and week out,
Your lucky charms will always let you down,
Maybe next week we'll see,
Maybe

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I heard if a dogs atacking you, pull its front legsapart andit kills it:|

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That's only if it's got lockjaw.

My dog rarely gets people because we stop him in time. It's just the fact he tries to bite them.

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Week in and week out,
Your lucky charms will always let you down,
Maybe next week we'll see,
Maybe

Also show him that u and the other person there get along. You may think that doesn't matter but in actuality if he's comfortable with u he may be more accepting of others =]

It's worked for me in the past so it's worth a try.

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Change for You - Midway State

Does he get excited and start barking when people knock/ring the doorbell? Does he go right for them when they walk in? Describe a typical scenario.

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"Men have forgotten this truth", said the fox, "But you must not forget it.
You become responsible, forever, for what you have tamed."

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Obviously, do not hit him or "tap" his nose.  

Does he get excited and start barking when people knock/ring the doorbell? Does he go right for them when they walk in? Describe a typical scenario.

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When the door knocks he barks his head off so we lock him away. But if someone comes in with a member of our family (ie they walk in because they have a key) he'll run straight for them and bark and growl and show his teeth, usually we push him away before he actually bites but there have been occasions where he has bitten people.

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Week in and week out,
Your lucky charms will always let you down,
Maybe next week we'll see,
Maybe

I fear what has happened is your dog has become aggressively fearless - perhaps from a lack of discipline (physical) in the past.

Though too much physical discipline can indeed also cause similar behavior. You have to find the right balance.

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Genetics is fate. Change our genes, achieve free will.

Maybe you could hold him on a leash in the house and have friends walk in or have someone come and knock on the door so you can try to control him a bit.  Praise him when he doesn't bark or carry on as much, reward him when he doesn't bite.

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Just because a heart is broken
doesn't mean it stops beating.