Theres really no scales for metal in general, only blues, asain, middle eastern, and your pentatonics and all the other crazy worded ones.

A good scale for metal which is also part of the Cowboys From Hell solo that sounds badass up to speed:

e|-----------------------|
b|-----------------------|
g|-----------------------|
d|-----------------------|
a|-----------------------|
E|-11-12-15--11-12-15---|
Pm.............

Do the same notes and pm up till the b string, on the b, you play the same notes, just don't palm mute, then play this on te high e:

e|--11-12-15--11h12h15p12p11-11h12h15p12p11---15^full step---|

If you get confused on how to play this, listen to the solo in CFH, is the fisr part that speeds up in the solo. You play it all as triplets.