#1
I have recently started getting back into metalcore (Parkway Drive, Bury Tomorrow, While She Sleeps, Architects) and am looking to begin composing music like this. 

However I am struggling to find what scales to play and how to work out what sounds good and I can never write riffs that sound good. Anyone got any tips on scales to use, techniques etc. 

I am tuned to  B F# B E G# C#  and I know how to do the basic string skipping for metalcore riffs just looking for scales to use etc. 
#2
Writing music is not about scales. I would suggest just listening to a lot of the kind of music that you want to write. Learn it by ear, internalize the sound and you will most likely start coming up with similar sounding stuff yourself. Few genres are about scales (blues is really the only genre that comes to my mind, but that's more of an exception because the blues scale is not a typical "western scale"). I mean, no single scale is going to give you a "metalcore sound". It has a lot more to do with arrangement (i.e., what kind of roles the different instruments have) and rhythms. You could write a metalcore song using the major scale, and you could write a metalcore song by using some more exotic scale.

But when I think of "metalcore sound", it's usually in a drop tuning and in a minor key. A lot of the riffs are something like
A|-----9---10---9---
E|-0-0---0----0---0-

So basically use notes from the minor scale and play the open 6th string between those notes. The audio quality is pretty bad, but this is exactly the sound that I'm imagining when somebody says "metalcore riff" (a video I found by Googling "generic metalcore riff"):



Also, remember to use a lot of sus2 chords. I'm talking about this voicing: x 3 5 7 x x (this would be Csus2 if we are in standard tuning). Also, most of the songs I have heard have been in a pretty fast tempo, but an important part of the "metalcore sound" is also heavy breakdowns in a slower tempo.

I'm really not that familiar with the genre, but those are some of the "cliches" I have picked up.
Quote by AlanHB
Just remember that there are no boring scales, just boring players.

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#3
This album basically invented metalcore:

There's no such thing; there never was. Where I am going you cannot follow me now.