If a sporter with a pivoting side safety on the receiver, 1st take down the action and make sure your tumbler has the extension for the safety - quite often, these were removed or the entire tumbler was replaced with a military one, which doesn't have the safety extension.
This is actually a pretty good safety in that is cams the sear up and blocks it, instead of just immobilizing the trigger. If a Greener GP action, the safety was self-setting - another reason to 'deactivate' it...