Small Arms Defense Journal spent some significant ink profiling the development of a new AR-15 platform compatible long-range round. The 6.5x40mm (aka .264 Warrior Magnum) is not to be confused with the (relatively) common 6.5 Grendel. Developed by Mitch Shoffner, the 6.5x40mm provides performance close to 7.62x51mm NATO at short range and exceeds it at long ranges.
The 6.5x40mm would require a bolt, barrel, and potentially a magazine change. The principal differences between this and the Grendel are the x40 uses the smaller 6.8 SPC case diameter (.42″ vs .44″ of the Grendel) but opts for longer case length. It operates from M4 magazines, but for optimal performance, would require its own purpose-built feed lips. Further details on the differences between this, the 6.8 SPC and 6.5 Grendel are in detail in the article.
As with anything, bullet design is an exersize in trade-offs. The 6.5x40mm is no exception, but for many shooters and military customers this benefits may be worth it. For a significant increase in range and simplifying logistics (no need for 7.62 DMR weapons), a 30% increase in cartridge weight, steeper cartridge drop, and small increase in recoil it may be worth it to convert.
In my personal opinion, the military is not likely to move from the 5.56 (especially with the new 77 gr. and new Enhanced Performance Rounds). Until the military decides to actually fight the next war instead of the previous one, development efforts of Wildcat cartridges will have to do for long-range AR shooters.
**All pictures courtesy of Small Arms Defense Journal.