The average load of a 7.62 x 39 has drop at 300 measured in yards, not inches. Just like the .30-30. That's over 36 inches of drop, and the western nations aren't going to take a huge step backwards to accomodate a few fanboys.
As for the popularity of the cartridges, the 6.8 SPC is an open format wildcat - everyone is cooperating and contributing improvements to the cartridge. The industry is now seeing 90 grain Barnes loads going out of 16" barrels at close to 3000 fps, and handloaders are definitely doing that. Check the posts on 68forums.com for the extensive list of ammo and makers of 6.8 barrels, uppers, and complete rifles.
The 6.5 Grendel is a great long distance cartridge, and holds the 600m record. With it's high BC and PPC cartridge case dimensions, it does a great job out of longer barrels. What has restricted it's growth is that it's a licensed product of Alexander Arms. There are less vendors and barrel makers willing to pay the fee to produce "Grendel," and as a result, there is only about 1/3 the ammo offerings and barrels.
Because of the restrictive licensing, lack of ammo, and a cartridge design that is optimized for long bullets, it's not breaking into the tactical carbine or hunting market with the success of the 6.8.
Both Italy and Czech Republic are looking at the 6.8 to be exactly what it was intended, an improved 5.56. Even Remington is offering the optional caliber for the ACR. That the barrel was marked " 6.8 NATO" may have been more wishful thinking than fact, but the impetus is there.
The major reason assault rifle cartridges were designed starts with the combat studies that show most infantrymen fire their weapons at targets under 400 yards. The Germans were the first to realize more powder or bullet than that was a logistical waste. The Soviet Union followed, and then America. NATO followed after. The average infantryman now carries a 4 pound lighter carbine and twice the ammo. (That's right, no net change, but the weapon is lighter.)
The calibers can be cussed and discussed, but the ones used worldwide are 400 yard cartridges in combat - not 600 yard paper punchers. Claiming the 6.5 is "just as good" out of a 16" barrel misses the point - it's not their best application. It's not for the .308, which requires an 18" barrel, and is considered inferior to the 6.5 according to some.
Because of all that, reduced recoil, and bullets designed for a higher KQ - Kill Quotient - the 6.8 is the fastest growing hunting caliber in America for medium game, such as whitetail or hogs. It's effective, flat shooting, and logistically supported by the industry with plenty of ammo available just as cheap as any other hunting ammo.
