It upsets me when my son draws this sort of stuff, because he always draws it on MY work! - I have a nicely drawn humanoid turtle concept (A turtle standing up), side views, good detail, really it was an inspiration to use when making the model. But, now my turtle man has a crudely drawn AK47 and a penis - not quite as inspirational.
I really don't mind what my son draws, as long as it's relevant. If he started drawing weapons for no reason then honestly, I would keep a closer eye on him. I think it's pretty clear that your dad simply does not realize why you would want to draw a weapon, he might not appreciate why you want to make a violent videogame either.
Now the arrogant part...
Have you thought that your probably gonna have to dumb things down so your parents can appreciate what your trying to do?
Think about how you can raise the subject and also discuss it with your parents without them coming to the discussion pre-loaded with concerns.
I would think about what you need to model, what you can practice on, and what you can do relatively quickly - but also something non-violent that your dad can relate to or even help with. For example, what if you asked him to help you model his car? - taking photo's rough measurements etc etc, then from that explain why you'd want a model of a car, weapon, girl with big boobs, whatever!
If you can show your parents that you have a talent in that area that could lead to a career, then you'll get more support and less hassle, simple as that. It doesn't need to be a car, just something that would interest him, then that would open up avenues for discussing these things and make it easier to breach the subject when you need something from them, like a new digi-camera or something.
