I am doing barbell shrugs as dumbbells has maxed out, but i am finding doing same weights with barbell is bit difficult as compared to dumbbell as my hands are in front. Is there any science behind it?
Friction between the barbell and your thighs effectively increases the bar's resistance to movement. I assume that the dumbbells do not rub appreciably against your thighs when you shrug them. So, you must work harder to shrug a barbell weight than to shrug dumbbells of the same nominal total weight.
Because the barbell is in front of you, its center of mass (COM) is horizontally displaced from your COM, whereas the dumbbells' COM does (very nearly) coincide horizontally with your COM. The barbell's COM's horizontal displacement requires you to to lean backward to balance the barbell's COM and your COM on a virtual lever about your ankles and above your mid-foot. Otherwise, you will tip forward, out of balance. This increases the balance challenge, and increases the friction between your thights and the barbell.