New Army "Caliber Configuration Study"

[stanc]

We're looking at 3lbs to worst case 5lbs added to the combat load.

Your calculations look good to me, but I ran my own using slightly different numbers. Plus, I expanded on it, to include a belt of machine gun ammo typically carried by riflemen.

The 30-rd, aluminum USGI 5.56 mag weighs 0.25 lb.

If the standard M4 mag well dimensions are retained, it's necessary to use stainless steel mags for 6.5 Grendel. I don't see the military regressing to 25 rounds in magazine capacity for general-issue carbines. IIRC, the 25-rd, SS Grendel mag weighs 5.5 oz, so I estimate 0.4 lb for a 30-rd mag.

300 rds 5.56 @ 190gr = 8.1 lbs
10 mags @ 0.25 lb -- = 2.5 lbs
Total --------------- = 10.6 lbs (+ 100 rds linked 7.62 @ 6.5 lbs = 17.1 lbs)

300 rds 6.5G @ 255gr = 10.9 lbs
10 mags @ 0.4 lb ---- = 4.0 lbs
Total --------------- = 14.9 lbs (+ 100 rds linked 6.5G @ 4.1 lbs = 19.0 lbs)