Originally Posted by
pinzgauer
Seem to have missed my point.
Yes, any caliber has it's sweet spot for BC. And sectional density is not the only factor in penetration (but it is a factor). And yes, SD only matters if it does not flatten/fragment (pay much attention to mainstream issue ammo lately?)
It's just that for 6.5, that sweet spot is close to the sweet spot for the combination of carbine recoil, flat trajectory, and downrange energy. With decent penetration, especially with (the probably required) lead free projectiles.
That "only 6-8 grains" difference is tangible performance difference when you are trying to optimize for a carbine with a particular weight/recoil profile while keeping downrange performance.
The USSOCOM update a few posts back is worth a scan. Nary a mention of 6.8 and 7mm, yet several 6.5mm references and not just about creedmore.
I'm a hardcore 7mm advocate for hunting, and used to compete IHMSA with a 7BR pistol. Love the 270. Still hunt with a 308, especially now that I can get decent BC bullets at lighter weight with excellent performance (TTSX).
But when constrained to the recoil/trajectory/ammo weight envelope they (big Army) are working with, it means lighter bullets.
The 6.8 bullet you mentioned with great vest/armor penetration sounds great, but normally sub-100g 6.8 projectiles are getting far out of the sweet spot for BC/trajectory and associated longer range performance (even accounting for lead free designs).
I know you are a hardcore 6.8 SPC advocate on your commercial site, but I personally do not believe big Army will adopt SPC or Grendel, or any other intermediate with brass case (7x46, 6x40) short of caseless that has a breakthrough which improves ballistic performance without compromising weight/ammo loadout & recoil to do so.
I'm a fan of the intermediate cartridge concept, but the logistical challenge and tradeoffs (15-20% less rounds for same weight) are just to big.
Especially with the improved performance they are apparently seeing with the new 77g and 855A1 5.56 loadings. Which buys them time to develop/push caseless/poly case.