Guys that like 6.5 grendel should try engaging moving targets with it and 5.56 and report back. 2600fps is noticeably slower than 3200. After you do that, compare them on a scale and remember an infantryman carries ammo around far more than shoots it.
I am a big fan of the 6mm grendels as their velocity is acceptable and think they'd be cool for special forces. But I'd still have big army running out 5.56 for the foreseeable future.
I’ve never been an advocate for replacing 5.56 with 6.5 Grendel, but for making something like 6.5 Grendel a DMR, Light Sniper, and LMG cartridge to replace 7.62 NATO.
If you run the numbers for Time of Flight with the same barrel lengths between a 5.56 shooting 77gr Mk.262 and a 123gr 6.5 Grendel
going 100fps slower at the muzzle, the ToF based on a doppler radar model looks like this:
77gr SMK 5.56 2650fps
50yds .06 seconds 1089ft-lbs energy
75yds .09 sec 1037ft-lbs
100yds .12 sec 987ft-lbs
125yds .15 sec 939ft-lbs
150yds .18 sec 892ft-lbs
175yds .22 sec 848ft-lbs
200yds .25 sec 806ft-lbs
225yds .29 sec 765ft-lbs
250yds .32 sec 726ft-lbs
123gr SMK 6.5 Grendel 2550fps
50yds .06 sec 1645ft-lbs
75yds .09 sec 1583ft-lbs
100yds .12 sec 1522ft-lbs
125yds .15 sec 1463ft-lbs
150yds .19 sec 1405ft-lbs
175yds .22 sec 1349ft-lbs
200yds .25 sec 1294ft-lbs
225yds .29 sec 1242ft-lbs (this is 42ft-lbs more than the 77gr SMK in 5.56 has at the muzzle)
250yds .32 sec 1190ft-lbs
I’m not sure what cartridge and barrel length we’re talking about for a 3200fps mv, as that hasn’t been a thing since we stopped shooting M193 from the M16A1 and M16A2. I remember getting M4s in 1997, when our unit transitioned from M16A2s to M4s, and never saw M16A2s in any of the units I was in after that. M4 with M855 averaged 2920fps per large sample sizes and lengthy Army studies. M855 62gr averaged a little over 3000fps from M16A2s.
With a really poor BC projectile going really fast from the muzzle, it will bleed energy rapidly even within the first 100yds. G1 BC on M855 averages .304, while 55gr FMJ (long obsolete from a US military standpoint) has a .243 G1 BC.
When talking about Time of Flight (one of the major considerations for shooting movers), BC does matter. The 6mm Grendel/6mm AR/6mm ARC will beat both the 6.5 Grendel and 5.56 77gr for ToF, while having less recoil energy of course than 6.5mm when comparing 105-108gr to 123gr.
The nice thing about Grendel-sized ammo compared with 7.62 NATO is that you can carry more for less weight, and have better practical performance in certain duty positions. Someone within DoD has already gone that route with the 6mm ARC, so they don’t have to lug SR-25s around as much.