Back in 2004, A-C-E posted a comparison of these two rounds, each of which can be fired from AR-style weapons with the same size receiver.
In "Special Weapons and Firearms" #42 (now on the shelves), the same author, Charlie Cutshaw, has another article about the 6.5 Grendel, in which he compares it to the 6.8 SPC.
I don't have a dog in this fight. I'm just reporting what I read.
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There's little argument that the 6.5 Grendel (derived from the 67mm PPC) has better long-range ballistics (beyond 500 meters).
"One fact remains incontrovertible, and that is beyond 500 meters, the 6.5x38mm completely overshadows the 6.8x43mm."
The debate has been around which is better at within 400 meters (the range for which the 6.8 SPC was designed, as a CQB round to replace the 5.56x45mm standard issue round).
Logic would seem to dictate a round with better terminal velocity and energy at longer ranges would also be better at shorter ranges, and a table [based on factory data for 20-inch barrels, not independent tests] in the aforementioned article supports this.
The 6.8 SPC 115gr leaves the muzzle at 2700 fps, and drops to 2417 at 100 meters and 1903 at 300 meters. Energy is 1861 at the barrel, 1492 at 100 meters, and 925 at 300 meters.
The 6.5 Grendel 123 leaves the muzzle at 2600 fps and drops to 2426 at 100 and 2098 at 300. Energy starts at 1846 and drops to 1607 at 100 and 1202 at 300.
At longer ranges, it's still all Grendel, as expected. At 500 meters, the Grendel retains 1797fps and 882 ft-lbs energy, compared to 1470 and 552 for the 6.8 SPC. The difference becomes more pronounced out to 1000 yards.
As Cutshaw puts it:
"In point of fact, the 6.5 Grendel's ballistics is superior to those of the 6.8mm at any distance."
The Alexander Arms site provides ballistics for the 6.5 Grendel zeroed at 600 yards. Here is a ballistics chart comparing, at a 300 meter zero, the 6.5 Grendel, 6.8 SPC, 5.56 NATO, and 7.62 NATO.
The comparison is not perfect, as the 6.5 Grendel has a 123gr bullet,
which the 6.8 SPC has a 115gr bullet (a 6.5% difference in bullet
weight).
Is the 6.8 SPC a better CQB round for other reasons than ballistics? Maybe. That's another discussion.
The author had access to only one commercially available load: the 123-grain Lapua Scenar HPBT (hollow point boat tail). But, its a good one, with a BC of .547. It remains supersonic out to 1000 yards. The average spread at 100 yards for two 5-shot groups was 0.65 inches.
Other cartridges are now listed for sale now at the Alexander Arms site: 90gr Speer TNT (BC 0.281), 120-grain Nosler (BC 0.458) and 129-gr Hornady SST (BC 0.485).
Most of the article discusses the Alexander Arms 6.5 Grendel, with an 18.5 inch barrel and an overall length of 39 inches (somewhat different than the prototype shown in the image above).
If you're interested in following 6.5 Grendel developments, visit the message board.
This is not the specific rifle nor round Cutshaw evaluated in the article
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