5.45 has lovely ballistics / a nice shape, and its essentially lead free (~95% of the bullet is either jacket or steel core.)

The French Balle D was reportedly solid brass.

And the worlds finest production bullets in terms of G7's are made of lathe turned copper:

https://www.warner-tool.com/flat-line-bullets/

17thfabn said:

Ballistics is it harder to design a projectile using the newer cores? Would it be easier to design the next generation cartridge if you could use lead?

Lead is dense and deforms mostly in plastic mode. Other metals (copper & copper alloys, steel...) are less dense and are more elastic.

So for a given bullet volume you have less weight and a lower BC (AEBE), which could be balanced by a better shape, but now people knows how to achieve very good form factor, even with lead-core bullets.

Current .308" 230 grs & 250 grs Hornady A-Tip have BCs similar or better than the original .375" & .408" Chey-Tac solid bullets, so any big .30" Magnum could now duplicate ballistic performance of those custom rounds that needed a custom action...

Accuracy could also suffer, because elastic deformations are reversible, so after "squeezing" the bullet into the bore, the bullet will expand slightly when leaving the barrel, adding a random radial velocity component to the bullet initial velocity.

You could use grooves or drive bands to reduce those issues, but drag will increase, along with dynamic instabilities due to unfavorable Magnus moment.

Lead-core bullets are very tolerant to barrel internal diameter variations, so your ".30 caliber" barrel could have land-to-land diameter between 7.54 mm and 7.65 mm, and groove-to-groove diameter between 7.79 mm and 7.86 mm, and still manage sub MoA accuracy.

"Solid" bullets are more tricky. You can use a solid bullet with a diameter of 7.81 mm in any barrel with a groove-to-groove diameter between 7.82 mm and 7.84 mm, but this bullet won't work in a barrel with a diameter between 7.79 mm and 7.81 mm (poor accuracy and fast barrel fooling), and sometime not in barrel with an ID above 7.84 mm because of poor sealing.

All those issues could be mitigated, but AEBE, bullets without lead core or lead liner (a small liner between the bullet jacket and a steel core, like the 7N6 bullet) are more difficult to design for a wide variety of barrel design.

In practice, we have seen low drag solids take off mostly in long-range calibers larger than .338 , and nearly all as single feed, even though some were advertised as compatible with conventional barrel twists and chambers, things quickly spiraled into much faster and possibly progressive twist rates and special chambers for solids and it remains the case today.

Indeed solids have to use 'under caliber' diameters, wasp waists or driving bands to engage the rifling and seal the bore and have in practice proven to be less accurate or at least harder to get the same accuracy. In meantime lead bullets also gained a bunch of new heavier weight options and narrower meats or sharper aluminum or plastic tips.

Its always hard to separate hype from reality but many lathe turned bullets are far from the consistency they advertise i have to find it but i have doppler measurements from an ELR competition where they measured 35 or so competitors and their shoots and i was quite surprised than many solids had considerable BC variation shot to shot Werner Bullets being the worst Cutting edge bullets being the best but lead cored Hornady was among the top performers

As mentioned above, when materials less dense than lead are used, the mass of a bullet particular shaped bullet goes down.  The ballistic coefficient is sectional density divided by form factor.   The only way reduce the drag of a less dense bullet is to improve its form factor.  This leads to attempts at highly optimized bullet profiles.   

That is more difficult than it appears because as a bullet travels, it is not pointed directly into the oncoming air.  Even a fully stabilized bullet travels at a slight angle to the oncoming air.  Before it "goes to sleep" or becomes fully stabilized, things are worse.  The bullets angle of attack to the oncoming airstream can vary widely as it goes through various oscillations.   The drag for many bullet profiles is increased significantly at the larger angles-of-attack that occur during these oscillations.   If I recall correctly, Emeric once stated that he has seen doppler radar numbers that indicate some bullets are still oscillating at 600m.  In other words, the drag measured from the flight paths of real bullets can be much higher than expected.  

When talking beter form factors in practice much of it is in the bullet tip. Note how many solids have extremely sharp tips.