I own the PAL copy of the game, being english myself lol..
and head popping is quite common.
with the rifle, its 100% as long as you can actually aim.
im currently playing thro it again, and a the point my shotty is about 3/4 tuned, and at close range heah popping is like 85-90%.
with the broken butterly, fully tuned, its almost 100%.
so as you can see from the small list of examples, the PAL copy must have the most gore, as if you have the japanese copy and no matter what you cant pop heads, there is a definate sensor..
altho on the bright side, some heads you pop, they dont die, the parasite starts to come out, and then you have to kill that.