I have an acoustic, and since I don't have the money for an electric I play metal on it as well. Due to (I assume) me switching between tunings, my G string started to break constantly (decent indent at bridge, which is plastic, and snapping at the nut). Deciding I wanted to rule out the possibility of breakage by tuning too often and too drastically, I have been on drop A (or A sharp, I honestly don't know) or C standard all the time, and I simply use a capo to switch between tunings. I have a few questions about this however...

First, is it safe to swap tunings often? If so, what's the limit?

Second, does anyone else use this technique? How did it work out for you?
There are several players out there who downtune & capo.

As for changing tunings...it's usually better if you can dedicate a guitar to a single tuning. This is especially true if you're working with a guitar that has a tremolo or if you need vastly different string sized in order to maintain proper string tension.
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Log off and play yer guitar!

Strap on, tune up, rock out!
Absolutely no problem with changing tunings often, your string breakage is probably due to too much tension (Try a different string gauge) or a dodgy nut (change it).

When you detune from standard are you going up or down on that string?
I think alternate tunings are amazing and I don't think there is a problem in changing them on your guitar - just remember to tune after putting the capo pressure on if you are using one and get a slide if you are using Open G (time for some Keith Richards )