I would agree, was it not that the prototype mount was dimensioned to the mm by oto melara art. that leaves 3 options: we measure the barrel wrong, the barrel goes on after the pivot point, or that reference of the prototype mount is wrong. in the first 2 cases your drawing might be too big, in the third the current drawing is wrong.
have you scaled with the official general arrangement plan? then it is certainly the third case. if it is concept art, of whatever origin, I thrust the plan on navweaps more and it is one of the first 2 cases. EDIT { and now I reread it was an pic. hmmmm that's pretty accurate, of course
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an idea might also be to check with the F125 class, which IIRC would get this gun as well.
sorry for being such a pain in the ass on this here miho, not that I do not thrust your research but there is something off here in one of the 2 drawings, which should be corrected (and if it is in my drawings of the concept, I will correct that myself)