“rechamber the weapon to fire 6.5mm grendal.”
Agree. The M4 itself seems to be a mighty fine weapon. The cartridge just doesn’t seem to have the punch. The 6.5 would be a good compromise, IMO.
The M4 in 6.5 gets my vote as well. They should keep the gas impingement system too as the piston system increases weight, decreases accuracy, and has very dubious claims to increased reliability.