As far as I know, when the BRL was first put into operation in 1943, due to the war effort, the priority was artillery shell modeling. So the standard projectiles put into use at that time were banded artillery shell types, like the Type 1, below, as well as were the Type 2, Type J3 (so close to Type 1, you don't see it used), Type 5, Type 6, Type 7 and Type 8. The BRL followed the late 19th century practice of using 1 inch diameter, 1 lb projectiles. That particular choice yields a ballistic sectional density of 1.0 so that the ratio of the ballistic coefficients of the standard projectile to that of a proportionally shaped projectile of another weight simply equals the second projectile's sectional density. For any non-proportional shape, this has to be adjusted by the ratio of drag coefficients of the two (a.k.a., form factor) at whatever Mach number you want the BC for. (A proportionally shaped projectile has the same drag coefficients, so the form factor is just 1, which is why you don't need to bother with it there.)
The spark photographs early in McCoy's book show un-finned projectiles as smooth sided. Some later ones in the book are of actual bullets, but those of experimental and standard shapes show smooth sides without driving bands. I am guessing that at some point after the war and perhaps as part of the numerous improvements they made at the BRL in succeeding decades, they may have gone to sabots to provide spin, but that's just a guess. They experimented with smaller rifled projectiles as part of the SARP program started in 1966 and did other things to see if smaller projectiles would fail to track the drag coefficients of larger ones due to their differing Reynolds numbers, the way spherical projectiles do, but didn't find significant issues.
McCoy always has his drawings in calibers so they can be scaled. These numbers are not always precise, however. In BRL-MR-3733 he has a sketch of the 173 grain M1 Type projectile with a 7 caliber ogive radius. Well, Hatcher shows it has a 2.1" nominal ogive radius, which is 7 times 0.300", and not 7 times 0.308". (You also find .300" used as the caliber in scaling targets for service rifle matches.) So, it's close, but not exact. That's OK for a couple of practical reasons: One is that forming dies don't turn out exactly consistent radii, so all bullets have some variation in this number even within a single lot number. From the drag table standpoint, dimensional differences as small as a radius difference of 2.100 vs 2.156 (7 times .308) make smaller differences in trajectory than are caused by velocity standard deviations and wind, so it's not really practically serious. Indeed, as Brian Litz points out, with experience you can guess a BC closely enough for general shooting purposes just by eyeballing the shape of a bullet and knowing its SD. Only at long ranges (800 yards and out) do higher precision numbers become important to keeping out of the transonic range and getting on paper.