The best choice to do what you want is by using the functions Quantity, UnitConvert and QuantityMagnitude.
UnitConvert@Quantity["ProtonMass"]
QuantityMagnitude@%
If you do not want or cannot use them for some reason, one possible way to get the numerical value from the measurement is by using Part or First.
Off[General::obspkg]
<< PhysicalConstants`
ProtonMass
%[[1]]
%% // First