So, I just broke the thicker E string on my guitar, for the second time since i've had it (only about 3 months). The other strings ive broken are the high e and the A string, and it seems odd that the thickest string breaks more often. It usually happens when im playing some metal and palm muting alot, so is this normal or am I doing something wrong, like putting the strings on wrong or something?

My guitar is a crappy Squier Bullet Special (i know, it sucks), if that makes any difference
More likely picking style, be a little less rough on them perhaps, its hard to tell without seeing it, you might be picking at a bad angle. You might also not be.
You don't need to neccessarily pick hard, so if you're playing it like you're trying to gouge out a hole in a metal sheet with a chisel, you might want to loosen up.

For the record: I've never broken a string on my guitar. (I've broken two on a friends, before I REALLY started playing guitar)
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Where is it breaking from? If its breaking nearer the bridge, then apply pencil lead in the bridge saddles(where the strings touch the bridge) to reduce friction. Do the same thing near the nut.

Otherwise, what gauge strings are you using? and what size pick? Maybe your just playing too hard? Plus what brand of string?
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My 7 String V build
My Main Guitars:
Kramer Striker FR-2027SM 7 String
BC Rich Afterburner Warlock
Washburn Xb100 Bass
My Effect(s)/Misc:
Digitech RP350
I'm using Dean Markley strings, not too sure about the gauge.
The string doesn't cleanly snap, it's just kind of stretched where the string is on the bridge.

I'm just guessing there is a problem with the may I palm mute, so I'll try to fix that