Military round comparisons

[LRRPF52]

You don't need a 140gr 6.5mm to have a .500 G1 BC. You should also be aware that G1 BC's are meant for flat-based bullets, not boat tails, and 5.56 and 7.62 have been using FMJ Boat Tails for a long, long time. The weight-to-BC ratio for a 7mm makes it too heavy, generating too much recoil, and too much cartridge space/bulk for a viable system that will be lightweight enough for riflemen.

There is some interesting work on a 7mm Grendel for hunting, but it is optimum with 120gr bullets. Those are much lower than a 6.5mm 123gr .510 BC projectile. If you're relying on just BC and trajectory comparisons, the 16" Grendel currently beats a 16" 7.62 NATO L129A1 or SCAR-17 for wind drift when pushing the 147gr M80, assuming a 2450fps speed for the Grendel, and a 2620fps speed for the 16" 7.62 carbine (7.62 NATO M80 varies in 16" guns from 2550-2620fps).

The 7.62 NATO at such low speeds for the bullet will not fragment at CQB (bad), and has the following drop/drift at 4400ft ASL (thinner air like A-Stan)...
Drop at 600yds: 4.6 Mils
Drift at 600yds: 1.4 Mils (10mph full value wind)

At 2550fps mv, it is:
600yds: 4.9 Mils
Drift: 1.5 Mils

The 16" 6.5 Grendel with current civilian factory 123gr at 2450fps:
Drop at 600yds: 4.9 Mils
Drift at 600yds: 1.2 Mils

The Grendel does that with 50% less recoil, much less ammo weight, less ammo footprint on the soldier's load-meaning more capacity to space. The energy difference at 600yds is 847ft-lbs for the 2550fps M80, and 763ft-lbs for the 2450fps 123gr 6.5mm.

Build a bolt based off of the .30 RAR, and you can push the pressure up to generate 2650fps in the 16" Grendel easily, as I have factory 123gr Lapua Scenar loads that have clocked over 2580fps in my 16" gun for some reason. Being moly-coated was one, and the temps on the range might have been another. You see the diminishing returns when you crank up to 2650fps:

600yds Drop: 4.0 Mils
600yds Drift: 1.1 Mils
Energy: 912ft-lbs (that's quite a difference actually)