Didn't read all the posts so sorry if someone already stated this.
I think you are arguing from the wrong angle.
1. You shouldn't argue that "gun control doesn't work."
The problem, is that in some cases it does work, and the other side of the argument will point out countries that do ok with strict gun control. In a debate, you've just been defeated on that point IMO. Also, you can't PROVE your argument, only make a conjecture.
2. You should be arguing that "more guns don't = more crime."
With this approach you can point out all the states and counties that do issue LTC. Then you can show crime stats and show that there has never been an increase in gun crime and violence after the onset of LTC policies. You can also point to other countries like Switzerland where all men are members of the militia and keep FULL AUTO rifles in there HOME. What's there crime rate look like compared to Great Britain?
Then you can move on to point out how some countries have enacted stricter gun laws and crime/murder rates rose. In this case, it's easier to concede that yes indeed sometimes gun bans work, but just as often they don't. On the other hand, allowing lawfull citizens access to firearms has never increased crime rates.
In either case, be prepared for the counter arguments. They are usually the same, but all of these can be proven not to be true with some facts.
Here's a couple links that might help:
Death by guns vs cars @ 1:20
Be smart, make sure that your argument can't be defeated. Don't ever make an argument that can be countered with facts, then you just look like them. That's a mistake a lot of people make, I even see it a lot on the forum. People make the wrong argument, or approach from the wrong angle and get backed into a corner.