I'm not sure I understand your question. If you want to measure to see if one is larger then the other use a piece of string. Rap the string around the bbls and mark it at where it meets the start. The one with the longer string has the bigger OD.
And what if it does have a larger OD? No biggie IMO... They may have different specs for the two different models.
------------------
Schmit
GySgt, USMC(Ret)
NRA Life, Lodge 1201-UOSSS
"Si vis Pacem Para Bellum"