Had a chance to compare the barrells of an MC18R2 and an MC21 (not sure if its an SP or not) for anyone interested. With a quick look they seem very similar.

Same overall height ie 86.3mm

Exhaust port seems the same size and in the same position within the bore ie approx 28.2mm from the E/P roof to the top edge.

Its the transfers that seem higher in the MC21. Seems to be about 1.5mm by my rough estimate which sounds a lot and may be my crap measuring.

Just using my fingers and not interested in the outright measurements just the differences so its rough.