Beng's explanation was spot on.
SD is a function of mass and cal. A 180 gr .308 cal bullet copper bullet has the same Sectional Density as a 180 gr .308 lead core bullet. The dfferense is in the density of the material (specific gravity). If both bullets have about the same form factor, the copper bullet will be a little longer and have a slightly lower BC because being longer, it will induce more drag. It will also require a tighter twist rate.
Just like lead core bullets, mono metal bullets have different terminal characteristics depending on the design and material of the bullet. They are made from different hardness of copper and the E-Tips are made of guiding metal very similar to jacket material. Some have tips and some have hollow points.
I am one of the few who do not have a personal preference based on what the bullet is made of. The bottom line for me is, 1) How well my rifle likes them - accuracy 2) Terminal ballistic characteristics 3) External ballisitic characteristics - BC to weight/SD to velocity 4) Cost
As of yet there is no perfect bullet for every situation.
The cup and core bullets in the 338 cal and down have a little better BC to SD factors than the monos. When you get above 338 cal, that starts to change.
Mono's tend to be more controlled expansion and are better at penetrating bone and dense muscle.
Cup and core bullets range in controlled expansion types (Bonded, Partitions, A Frames, etc) to highly frangible (Bergers, Amax, etc). The more frangible, the more damage they will do in soft tissue like lungs and liver but the less likely to penetrate dense muscle and bone.
I do lean more toward controlled expansion thinking, but the vast majority of my shots are lung shots and the best bullet for that is a Berger. But... in the rare case i have to make a quartering shot through muscle and bone, I would feel better with a controlled expansion type bullet. The question is, which is most practical overall?
I used to load E-Tips for short to mid range hunting and Bergers for longer range.
Of all the mono's I like the, the GSC HV bullets the best for terminal ballistic design but I could not get the 308 177's to group less than MOA out of "my" rifle. There is very little room for seating adjustment with them and unless you have a very short throat they make a big jump to the lands.
In my new 300 RUM, I will be trying 230 Bergers, 200 CEB's and 177 GSC HV's. All these bullets will perform well terminally to well past 1000 yds. The 230 Bergers will have the clear advantage past 1K. Accuracy will be the first consideration in selection. I may even go with a 2 load line up again to cover all the bases.
CEB terminal performance
Cutting Edge Bullet Kills
Cutting Edge Bullets terminal performance
*Updated 10/29* Terminal Performance 6.5mm 130gr CEB
GSC terminal performance. The GCS conceept is lighter bullets, higher velocity which doesn't work well with cup and core bullets.
GS CUSTOM BULLETS - FAQ - Expansion and Weight Retention
Of the mono metals, I like CEB and GSC the best.
Advantages and disadvantages of each.
CEB's seem to be easier to load for accuracy, but my experience has been limited.
CEB's are about 2/3rds the cost of GSC's
CEB's are quicker to order
GSC's probably have better terminal performance including lower opening velocities down to 1600 fps. However, the lower opening velocity is due to a larger hollow point meplat opening, which degrades BC. I base the terminal performance assessment on what I've read, no first hand experience yet.
GSC's can get a little more velocity than CEB's of same weight based on my experience, about 30- 50 fps more
Both CEB and GSC will make custom projectiles to your specs.