What is 120 300mm on aps c in full frame?

120-300mm on APS-C gives the view of 180-450mm on full frame.

The same view on every common format:

• Medium format (44 x 33): 228-570mm
• Full frame: 180-450mm
• APS-C (yours): 120-300mm
• APS-C (Canon): 112-281mm
• Micro Four Thirds: 90-225mm
• 1-inch: 67-167mm
• 1/1.3-inch: 50-125mm
• 1/1.7-inch: 39-98mm
• 1/2.3-inch: 32-80mm

Crop factors are by the sensor diagonal, rounded as they're usually quoted; a format with a different shape (4:3 against 3:2) frames a little differently across the width. More in the focal length converter.

Computed from the sensor sizes, not written by a model.