As all European guns are now proofed in metric sizes they will be-

18,3 , 18.4 . 18.5 etc. for 12 bores that is equivalent to-
.720" , .724" ..728" etc. There may be a slight discrepancy in these equivalents but close enough. With .2mm or.008" between proof sizes .
Many makers now sell " back bored guns " which basically means over bored and these come in at any thing from 18.7mm /.732" to 18.9mm/.740" [average 18,8mm/.736"]