02-11-2013, 09:45 PM
LRRPF52 Wrote: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)
Most of the data that I have refers to the 6.5mm bullet at 140 gr. When looking at a potential GPC, 6.5mm is considered to be the bottom of the range and 7mm at the upper. The 6.5x55 is seen as the gold standard by which everything else is compared. It pushes a 140 gr. bullet at 2500 fps which does more damage down range than does a faster bullet. With a BC of .500 or better, the bullet retains more velocity, thus energy, longer.
Frankly I am not seeing much gain (over the 6.5 Grendel) with the 260 Rem (.308 case) or the 6.5mm BRM (based on the .30-30 case) let alone many of the other rounds. If we come up with a General Purpose rifle in 6.5mm, it should be able to use a bullet from 120 gr. to 140 gr. if needed. From what I have seen, the Grendel does as well as the 7.62x51. I would think that the G7 bullets would be better than a G1.
The three things that a GPC would need is a BC over .500, muzzle speed of 2500 fps and potential to be used in the M16/M4 platform. But my question would be whether there is ANY advantage of going to a larger caliber (ie 7mm), of using a longer case than what we have in the 6.5 Grendel. How close is the Grendel in potential to becoming the GPC?