I tried to do megashots a few times, wouldn't let me for some odd reason.
It might be because you're trying to set it to something ridiculously high such as 10x for example(and just an example since 8x seems to be the upper limit for megashots) at 1920x1080 for a 19200x10800 image which is obviously absurdly large. It seems to not let it go past 3x for a full HD monitor at least for me, but just try 2x or 3x, since much past that tends to not to work for some reason. You might be able to do it more than 3x, but 3x is the apparent limit for me at full resolution, so 2x or 3x should work fine.
so now we have ultra shader megashots. idk it just sounds cool, and I bet they look cool too. I've never tried one
Megashots from my experience don't seem to look too great at the pixel level, but that's probably just me.