The main disadvantage to basing the Intermediate Cartridge Series on the AR-10 (.308/7.62×51) platform is the higher gun weight as compared to the AR-15. But let me say two things about that.
First, this disadvantage is outweighed by many other factors. The ability to have a longer COAL results in longer high-BC bullets, heavier bullets, more powder capacity, higher velocities, and, with the narrower diameter case, more rounds in any given size magazine as compared to the AR-10.
Second, the AR-10 platform is getting lighter. As more companies make their own version of a “modern sporting rifle” or AR or 762 or whatever they decide to call it, they find ways to make the gun lighter. The following is a list of gun makers and their stated weights for AR-15 (.223) and AR-10 (.308) type rifles. Rifle features and barrel lengths vary, so these specs are only a guideline. Unless otherwise stated, the weight is for a rifle with a 16″ barrel.
7.2 lbs in .223 (14″ barrel)
7.9 lbs in .308
Heckler & Koch
8.40 lbs in .223
10.16 lbs in .308
7.9 – 8.5 lbs in .223 (14″ Tavor)
8.16 lbs in 7.62×51 (16″ ACE)
7.3 lbs in .223
9.5 lbs in .308
7.7 lbs in .223
8.7 lbs in .308
Robinson Arms (XCR)
7.5 lbs in .223
8.2 lbs in .308
Rock River Arms
8.4 lbs in .223
9.1 lbs in .308
7.94 lbs in .223
8.60 lbs in .308
6.74 lbs in .223
7.71 lbs in .308
6.4 lbs in .223 (lighter config)
7.25 lbs in the new GII (.308) with a 16″ barrel;
7.76 with a 20″ barrel
7.8 lbs in .308 (CA-10 DMR)
The selection of .223 rifles above range in weight from 6.74 to 8.4 lbs. But the .308 rifles have an overlapping weight range from 7.25 to 10.16 lbs. Some AR-15s are heavier than some AR-10s.
Of course, that is for an unloaded weapon. Add a full magazine, optics, bipod, muzzle device, and whatever else, and the weight quickly rises.
But when we convert an AR-10 style rifle to ICS, there may be some weight savings. The ammo is lighter, but more rounds fit in the magazine, so that may end up being a wash. The barrel can be shorter in ICS than in 7.62×51. There is less powder to burn, and the cartridge is efficient, similar to the 6.8 SPC (which likes short barrels).
The lower recoil of the ICS might allow for some weight savings over the 7.62 in the bolt carrier and buffer system. An AR designed from the ground up for the ICS would offer the opportunity to reduce weight based on the lower recoil.
On the other hand, the AR-10’s are typically about a pound or less heavier than the corresponding AR-15 by the same company. I’d say that it’s worth the extra pound to have higher bullet weights, better BCs, higher velocities, and greater range.
I would also like to point out that the Israeli Defense Force’s Tavor 21 weighs (in various configurations) from 6.5 to 8.1 lbs. But they also do not add as many military gadgets to the rails on the rifle. So with a heavier rifle, they may end up with a lower total weight.