11.5mm seems like a really oddball length. 12mm is the closest common length, and 0.5mm (.020") isn’t much. Have you already confirmed that a 12mm long screw won’t work? If so, grinding the tip down slightly with a rotary tool or bench grinder may be your best bet. If you can, thread a nut onto the screw before you grind it down. That way any threads that are messed up by the grinding will be restored when you take the nut off.
11.5mm seems like a really oddball length. 12mm is the closest common length, and 0.5mm (.020") isn’t much. Have you already confirmed that a 12mm long screw won’t work? If so, grinding the tip down slightly with a rotary tool or bench grinder may be your best bet. If you can, thread a nut onto the screw before you grind it down. That way any threads that are messed up by the grinding will be restored when you take the nut off.