Although I don't agree with the American Second Amendment on principle, I'm going to disregard that belief for this topic. But of course it should be noted that my true reaction is that the Second Amendment is wrong. (This is not the point of the topic though, so no quoting me on it. I'm just stating this so people know where I'm coming from)
As a rule I believe that you should respect the rules of the house you live in. If you live in your own house, you set your own rules, but if you want to continue living in someone else's, whether it be family, friend, rented accomodation, then you must abide by the rules set in place by that house. Otherwise you should leave. If you can't leave (for reasons of age, monetary etcs) then you simply have to accept your place in life. You can't change things simply by beating your head against them, that's idiotic and no good is likely to come of it. Once you are independent you can do whatever you like, but even then if you were to break the law there would be consequences. House rules are simply a way of preparing you for life.
Point the second: Training aside, for target shooting or not, guns are not only dangerous, but they are tempting. If someone is robbing you, and you have a gun, there is a temptation to believe you are immortal, and that you can take the burgler because you have a gun. Then you are placed in a position of possibly being killed, or killing, another human being, and that is something which should never be taken lightly, whatever their offence. Having no gun, while it might mean you personally cannot defend yourself, removes the temptation to take the law into your own hands and enact a legal killing. (This point is of course dependent upon circumstance, but even so, it has to be addressed)
Finally, although your guardians don't agree with you, their opinion is still just as valid as yours. Having a reasoned debate with them is fine, but beliefs about gun control are often strongly held, and as said above in my house rules point, it seems to me that wanting to challenge the very nature of your relationship with them, over something as relatively inconsequential as a rifle, doesn't strike me as being a sensible and mature course of action. People get kicked out of their house for wanting to own a console or an instrument, so to bring a weapon into it just ups the ante. In the end what you are asking them to do, one way or the other, is accept that their teenage child wants to own and keep a lethal device with them around, and I honestly can't blame them for being antsy about this.
So in the end, they sound a little over-anxious (no good will come of it. At the very least nothing will change, but I do agree that I can't see anything necessarily good arising from this change in situation), in that if they seem to believe that you owning the weapon will mean you become the next Columbine killer, then they are overreacting.
HOWEVER
It is their house, it is their rules, and it is their beliefs against yours, and disrupting all that over a gun is not worth it. You may both be in the wrong, but for better or worse, you're the one who should in my opinion back down over this.