The 7mm designation is a reference to the bore size, or diameter. In decimal size it means this rifle has a barrel groove diameter of .284 inch.
The carat size of a diamond should be determined by a certified jeweler. There are two different sizes the is the millimeter size which as stated was 7mm and then a jeweler can give a carat size.
The word passable is an adjective. It describes something that can be passed.
An old mine cut diamond that measures 7mm possesses one measurement that is 7mm. No other data about the diamond can be deduced about the diamond given this measurement.
7mm
7mm.
7mm
7mm
7mm Hex.
7mm Allen wrench
7mm Allen
A 5mm lead is darker than a 7mm lead because the larger the lead size, the lighter the mark it tends to produce on paper.