Jadegold has recently released a string of articles on how to debate with gunloons.
I too was on that side of the fence at a time. Oleg Volk changed that, but I can still build a case in favour of gun control, arbitrairy as that case might be. Today I will at least try tell you what NOT to do.
First of all you should get familliar with the issue. Failure to be familliar with the issue may result in a complete loss of credibility and talking around the issue will not convince anybody. Typical mistakes incorporate using hollywood slang for technical firearm terminology. The most common mistake is using the words magazine and clip as if they were interchangeable. They might be in real life, but not in legislation. Being able to distinguish a revolver from an automatic pistol, knowing the difference between a (semi)automatic or selfloading weapon, and a fully automatic one is the bare minimum.
Knowing what an "assault" weapon is, is another can of worms alltoghether. It's a risky endaveour calling any weapon an assault weapon, so it's best to refer to a a gun by its action (bolt action, semiautomatic, fully automatic, pump action, ...) The Brady campaign originally defined what an assault weapon was, but has over time they have labeled other weapons as assault weapons, even though they were not subject to the assault weapons ban, or would have been if the ban would still have been in effect. To make matters even worse, there are other definitions as well, such as military or corporate designations, as well as other names for the same weapons, and the police might call their own something entirely different still, like patrol rifle (and that is in turn a way of referring to one specific rifle, the FN patrol bolt rifle)
It's easy to get caught up in word games, and somebody who's in contact with firearms on a daily basis is likely to have the advantage over you on that front.
It's very important to realize that the gun rights activists have more statistics to support them that any gun control activist can ever dream up, primarely because gun control means change, and even when statistics only show irrelevance, it supports the arguement that gun control is not needed. Both the Brady campaign and the violence policy center provide a vast amount of statistical data, but like I said: The gun rights groups have more, much more. In the gunfacts file you're likely to find out what's wrong with those numbers. Gun rights activists know these texts all too well, and if you're sitting down on a debating table expecting to baffle your adversairy with numbers, you'll likely have all your arguements turned against you.
To illustrate that: The US is has the best armed civillian population in the entire world, and one of the highest homicide rates as well, there is however no absolute correlation between gun ownership and homicide rate, or violent crime rate. So, never say that guns cause crime, you cannot prove that, just like your adversary cannot prove that guns reduce crime. The only way to deal with this issue is to shift the burden of justification, demanding why people insist on keeping arms. This will land you in an arbitrary debate, your wishes versus theirs, a debate without end.
Never say that there wouldn't be any mass killings without guns, you'll be met by a mention of the Oklahoma city bombing or the mass school stabbing in Japan. Instead say that a lot of people might still be alive today if it weren't for guns. This puts you in the field of tealeaves, becaue we have no way of telling if a killer would have carried through with his plans if there had been no guns at his disposal.
Never blame a specific type of gun or call any gun "better than another". The recent shootings in Nebraska and Colorado have rekindeled the discussion on assault rifles. Cho, the gunman of the VTech massacre didn't use a rifle to be the bloodiest school killer in the history of the US, so don't put the blame for mass shootings with these weapons.
Avoid bringing up other countries that have succesfully implemented gun control legislation, anybody who debated seriously in favor of gun rights will be able to cite several government studies showing that such legislation hasn't made their population any safer.
Don't try to make a case in favor of less than lethal weapons, there has been a string of fatalities tied to such weapons recently. Furthermore, a lot of these weapons are already banned under state law in a lot of places in the US (PAVA capsules for example are illegal in six states). Also, a gun rights activist is likely to expose these as not a valid means of self defence, as well as bringing up the recreational use of guns which isn't replaced by these alternatives either.
Maybe I'll type some more later.
And before you say it, yes I know you see what I did here.