Unlike the 6.8, the 6.5 came from the fertile mind of an inventing genius who wasn’t trying to invent a better jihadhi-busting round. Bill Alexander is one of those guys who can’t look at a part, mechanism, cartridge or other design without thinking of ways to improve it. The 6.5 Grendel is a simple-appearing cartridge. To define it in the simplest and most Bill-discounting terms, it is a 6.5/7.62X39 Ackley Improved. That is, it is the Soviet 7.62X39 case, necked down to 6.5, and with the shoulder blown out and sharpened, and the case walls straightened.
Which grossly diminishes the work necessary to refine the dimensions of each. To give you one example, the case neck: how long? A shorter neck means a more-forward shoulder, and thus greater case capacity. More capacity means more powder, leading to more velocity, and greater range.
However, a shorter neck also means a less-pointy bullet, and thus a lower ballistic coefficient, leading to velocity drop at range. A shorter neck also means less tension on the bullet, and a greater likelihood of bullets loosening on feeding (being rudely shoved up the feed ramp) and a blown case from bullet setback. Some like to compare the 6.5 to the 6.8, and start an argument as to which is “best.”
They are more alike than they are different, despite the cases being so different. The two each start bullets in roughly the same velocity range, with bullets of similar weight, and the close-in performance is similar. (And both sides will hate me for saying so.)
The difference is in the long-range performance. A 6.8 bullet of 110 grains that starts at 2550 fps reaches the 500-yard line with 1515 fps and the 1,000-yard line with 980. A 6.5 Grendel, launching a 123-grain Lapua Scenar at 2650 fps, reaches the 500-yardline with 1890 fps and the 1,000 yard line with 1304 fps still on board.
For pretty much the same shoulder-thump, you get far better downrange performance, once you exceed the “typical combat” ranges of 300 meters.
The 6.5 Grendel requires the same parts be exchanged to create it as the 6.8: bolt, barrel and magazines, although none of the three is cross-compatible between the 6.5 and 6.8. You pick one or the other, not something that does both.
Starting out, Bill Alexander patented and trademarked the cartridge and components, because the performance and accuracy were the big advantages of the system over other calibers. To devise something that performed and then allow anyone who wished to, to make one, and potentially diminish its performance and reputation,was not what he wanted. He has since licensed the round and designs to others.
Do you need a 6.5 (and when has need ever entered into the discussion)? Well, if you want to do long-range precision work, and don’t want the bulk and thump of the .308 in an AR-10 type rifle, yes. You can shoot to distance with a 5.56; the NRA High Power ranges have proven that. However, at 600 yards the 5.56 is not exactly the hammer of Thor. If you want to reach out and have some tap left, then the 6.5 is the next step up.