Because there is not a mandatory sentence for carrying a knife does not mean that
Because mandatory sentences are not the best way of doing things.
The whole point of having a judge is that someone sits through and listens to the evidence of a particular case before deciding on a sentence rather than making generalisations.
Setting mandatory jail sentences would end up sending to prison a lot of kids who still have a chance to turn their lives around. By the time they come out they won't and will most likely put to use everything they learned inside.
That's why you need to give judges the discresion to sort sheep from goats.
Gun crime is different. Guns aren't carried by kids on the fringes. The average age is mid-twenties and they've been well involved with crime for a while before they get a gun.
actually they look like this:
http://www.homeoffice.gov.uk/rds/pdfs06/hors29 8.pdf