Should be alright but the 12 to 28 makes more sense and it easier. Why take any risk when you do not have to? The 12 to 28 I am sure will work and be fine while the 12 to 20 should be OK. Not very much metal in the forcing cone area of a Chambermate turned down to fit a 2" chamber. Would not be surprised to have one crack if you remove too much metal. They will be very thin.