Hmmmm...
I'm kind of late to this conversation, but I'd like to throw in my two cents worth.
In my experience, the best way to counter people on any subject is first to know the subject well yourself. And then lead the other person by drawing information out from them first...information which will support your viewpoint which you will state later. Feel free to take them down the odd tangent once in a while, too. But make it a VALID tangent.
Going head-to-head with many of these people is, in most cases, less than useless. They have their own set of facts and data and they'll stand safely behind them to the end. But getting them to open up by drawing information out from them produces chinks in their armor.
Don't get me wrong...most will still stand by their own point of view. But you have a much better chance in defending your own point in this fashion.
We all know numbers play an important role in this, for example. So you might go with that. Here is an example:
YOU: So, you want to outlaw magazines with greater than 10 round capacities?
HIM: Yeah. If nobody had high capacity magazines, then these kinds of things wouldn't happen.
YOU: Huh. How many gun owners are there in the country?
HIM: There are about 75 million people who own guns in the United States.
YOU: Wow! That's a lot of people! They all own high capacity magazines for their guns?
HIM: Well, most of them probably do. Obviously shotguns don't have high capacity magazines.
YOU: How many murders are commited each year by guns with high capacity magazines?
HIM: Well, over 30,000 gun related deaths occur every year.
YOU: But how many of them were murders commited by criminals with high capacity magazines?
HIM: Well, about 10,000 or so were murders. But most of the others were suicide.
YOU: Hmmm...I think we can safely say that high capacity magazines are not a factor in suicides, don't you?
HIM: Well, yeah, I suppose so.
YOU: So, out of those 10,000 murders, how many were commited by known criminals? Criminals who could not legally have owned a gun in the first place?
HIM: Probably most of them.
YOU: OK, let's go with "most of them". Let's assume that there were 7,000 separate criminals involved in the killing of those 10,000 people. Sounds like a lot!
HIM: Yeah!
YOU: So, let me get this straight: 7,000 known criminals, who already aren't allowed by law to own guns in the first place, killed an average of about 1.5 people each.
HIM: Well...
YOU: And a 10 round magazine limit changes this how?
HIM: I'm not sure what you mean...
YOU: Well, let me ask you another question...you said that there were 75 MILLION gun owners in the country. These are LEGAL gun owners...law abiding citizens. These people don't seem to factor into the actions of these 7,000 criminals, who each took an average of 1.5 lives each. How is making it illegal for THEM to get "high capacity" magazines stopping these criminals from taking 1.5 lives each?
HIM: But, but, but...
This is just an example. The object is to get THEM to volunteer enough information so that YOU can turn the tables on them LOGICALLY and USING THEIR OWN INFORMATION. And, much like this site, take the high road when dealing with them. Do NOT engage in character attacks or belittle them. Treat them with good manners and keep it on the level of a debate, not an argument.
And choose your battles. No one can take up every challenge that comes his way and realistically expect to win them all, so keep your "wins" high by choosing wisely.
This is the only hope you have of reaching even the few who will listen to you.
Oddly enough, I've won the respect of many an opponent this way. Not that I changed their minds, necessarily, but respect for the way they were treated and in keeping the dialog from devolving into an argument. Sometimes and "agree to disagree" will be the best you can hope for.