[B.L.E.]

The shape of a bullet also has an effect on the BC. A spitzer boat tail usually has a higher BC than a flat base flat nose bullet all other things being equal.
The smaller the bore, the harder it is to get a high BC. Even the heaviest .17 caliber bullets have BCs in the 0.125 range.
The reference bullet that defines a BC of 1.00 is actually a small artillery shell with a 1 inch bore and a length of three inches if I remember correctly.