How do I get that growl in my singing voice?

I normally sing Avenged Sevenfold and Bulllet For My Valentine but I also do screaming too so my voice is kind of raspy. But what I'm wondering is how do I get the growl like James Hetfield (Metallica) and Matt Heafy (Triviium)? I don't want to get a teacher because all I've done the past couple years is look online for tips on screaming and singing and it has worked for the most part. This is the one thing I can't find though.