YOU SAID:

Put a gun against his head

INTO JAPANESE

彼の頭に銃をかける

BACK INTO ENGLISH

I put a gun on his head

INTO JAPANESE

私は彼の頭に銃を置いた

BACK INTO ENGLISH

I put a gun at his head

INTO JAPANESE

私は彼の頭に銃を置いた

BACK INTO ENGLISH

I put a gun at his head