In politics, using English, the expression through the barrel of a gun means only one thing: through the use of armed violence; the opposite of peaceful, lawful politics.
Example: (Warning! Quotation does not imply endorsement of claims) (Subject: Sudan) "Abdullahi al-Azreg, Sudan's ambassador to London, dismissed predictions of looming mayhem as insulting and exaggerated but admitted there were serious problems. ..."The SPLM is ruling the south through the barrel of a gun. It is intimidating the voters," he said. "The last election [in the south in April] was not fair, it was not transparent, it was rigged. If the referendum is the same, we could not accept it, we would reject it. If there was fraud, we would say so straight."
This is to say, the south of Sudan is (according to this ambassador) being ruled through (in his view) illegitimate armed violence. When "the barrel of a gun" is used, organized armed violence is usually implied.