Because the handguard is 8" presently, and I have a flash hider that reflexes the can over the barrel. By using essentially a 9" handguard, there isn’t enough clearance to run the same flash hider/ mount. So the other standard mount would increase the OAL, not the handguard.
I’m thinking now that I should change to the Larue on that 10.5" 6.8 pictured above, and use the old handguard and reflexive mount on a 10.5" 5.56 and run a SCAR-SD with it. I guess it’s kinda silly to shorten it up with a full size rifle can on the end of the 6.8.