My band and I are planning to play some Slipknot and maybe Korn (A) but we don't want to get 7-s guitars so what string should we use?
Ah, I had to do this very recently, I grabbed a set of Ernie Ball Not Even Slinky's:
and they worked fine on my Telecaster.
.012 .016 .024p .032 .044 .056
I actually went a step further and tuned down to A on my Tokai Telecaster and it seems fine, a little loose, but doom all the way. Hope that helps : )
Epica and Carcass, two bands that have guitarists that tune down to B use .13 strings (though I can't seem to find the respective interviews right now, I'm pretty sure of the fact).
That being said, I tuned a guitar down to B once with .11 strings. It's not the best option (playing gets too light), but it's doable. For metal, I'd recommend going for the .13 though.
I use .13-56's on my Jackson King V and they work beautifully in B, I've even gone down to A with them no problem. I use Dean Markley DT strings if that helps you.
Buy a 7 string set and use only the 6 lowest strings. They'll be the correct gauge.
Note that you'll have to adjust the intonation after a big string gauge change like that.
The biggest question that you'll need to answer is how you want to tune the top two strings? You can tune them the way the 2nd and 3rd strings are tuned on a 7 string (B and G) or like a baritone guitar (B and F#). The advantage to the baritone guitar way is that all the chord shapes you are used to will work, although the chords will all be different (for instance, an E chord shape will give you a B chord tuned that way).