I reload for 6.5 CM and I cant see how Grendel would be cheaper. Creedmoor brass is cheaper and much much more plentiful. bullets are the same. The CM would just us some more powder.
As far as confined environment, Id rather fire a 16 inch .308 indoors than a 10.5 inch 5.56. My 10.5 inch SBR is a concussion canon. I cant imagine that a .308 is any louder than a Creedmoor.....besides, thats what suppressors are for.
I dont see how 6.5 CM could be considered too much for an assault rifle, its more mild than a .308 in kick and muzzle blast and there is no doubt you can get better velocity out of the same 6.5 bullet and barrel length with the CM than the Grendel.
Add to the fact that very accurate factory 6.5 CM ammo is readily available and Grendel ammo is harder to find than hens teeth, the CM would be a much better 6.5 choice. Availability of factory ammo should be the biggest deciding factor. Besides, from what I have seen several times, the Grendel round would have been better off as a 6mm cartridge anyways.
I get 2850 FPS with a 123 Amax out of my 6.5 CM gas gun with a 24 inch barrel. 2600 would be doable with an 18 inch barrel and in that short little package that would be a devastating combo.
I have both a Grendel and a .260 Rem, both in AR family weapons. The .260 Remington is not an intermediate cartridge, it falls into the battle rifle cartridge category, due to size and recoil. The shot recovery with any of the .308-based cartridges in a little bullpup is going to be problematic, as is weapon longevity in such a lightweight package, unless you use constant-recoil or other mitigating mechanisms that don't involve impact of reciprocating parts. You're dealing with 60,000-62,000psi SAAMI cartridges with .260 Rem and .308 Winchester.
I'm really interested to see the MDR in Grendel, since the design has room for a larger bolt. A little 18" or even 20" Grendel would be sweet for a DMR.
As soon as I looked at and handled the various prototypes at SHOT last week, I started thinking that the bullpup code might actually have been cracked with this ejection system and ambi mag release by the firing hand. The trigger felt nothing like a pup, but it wasn't resetting, so there is some work to be done between now and being truly ready. I think the main thing they pulled off was proving the ejection system, overall layout of the gun, and basic functionality with the prototypes.
It will be interesting to see this project move forward. From manufacturing and end-user perspectives, lower pressure cartridges make a lot of sense. Shot recovery, accuracy, and recoil. Shooting from positions is much easier with a pup since you don't have forward weight away from your C of G.
I'm going to be doing some demonstrations with carbines vs. precision rifles with more accuracy potential in terrain on a shot timer, showing the advantage that smaller guns have. For example, a 14.5" AR15 lightweight carbine in the same shooter's hands will have effective rounds on-target faster than an 18" SPR type gun. I suspect that a pup will allow even faster in-position times with rounds on target. Take away the barrel length advantage with a pup, and you have a very solid platform to deal out fast mv's from difficult positions.
Very rarely do you actually have the option to shoot effectively from prone in the field.