I noticed that sometimes the pronunciation changes when combining words. An other example is 不死斬り (ふしぎり) and 腹切り (はらきり); both 斬り and 切り are pronounced きり independently but only in 不死斬り is 斬り pronounced ぎり and not きり.
Is there a rule for that?
My assumption is that it depends on the ending sound of the first word. If it ends with a i sound (ni,shi,etc) then they lead to dakuten (") on the following syllable.