Based on my interaction with secret service agents, I don't think you'd be at much risk of getting shot unless you looked like you were making a move toward your gun.
They're extremely professional. Smart, very well-trained, and intensely focused when on the job. They know how to prevent threats without escalating to deadly violence, as evidenced by the distinct lack of secret service shootings, beatings, etc.
They are also very thorough and I guarantee that you wouldn't get in with your weapon. If you weren't actually interested in going to see McCain, but just in finding out how the secret service handles such situations, and in testing to see what they'll do if you exercise your right not to talk to them, it could be educational. AFAIK, they have the right to keep you out of the event area, but not to detain you, disarm you or search you. I'm not sure whether or not they can legally require you to identify yourself.
In short, if you were to try this, you would only end up testing how good the secret service is at properly observing your legal rights. If they're not very good at it, you may be in for a very unpleasant experience. I suspect they'd handle it well, but that's just a guess.
If you were enough of an activist to be okay with the risk (for little, if any, gain) then my suggestion would be to first consult your attorney to be sure that you know exactly what your rights are so that you can stay scrupulously within them. Bringing your attorney with you would be a good idea. Also, make sure you have the next few days off work, and some money to pay your lawyer because even if you're completely in the right, things could get a little bit ugly, and I wouldn't be surprised if the judges were very unfriendly to your cause.
I don't think you'd really be risking life and limb to try this, but you'd be risking a lot of inconvenience, expense and legal hassle.
Oh, and McCain would probably never even hear about it unless it made the papers, which would probably be very bad.