Standard copper fittings fit K,L,M. There is no difference in the OD of these different tubes. You mentioned type ACR: the difference in the tubing is that ACR is referred to by OD; i.e. 1/2" type ACR is actual 1/2" OD. When we refer to standard hard or soft copper tubing, type K, L, or M the callout size is NOMINAL ID. The actual OD is excactly 1/8" larger than the callout. SO, 1/2 type K,L, or M all are exactly 5/8" OD. The wall thicknesses are different so the actual ID are different, but all are referred to as NOMINAL 1/2.