6x43 is what the Small Caliber High Velocity Infantry Rifle should have been in hind-sight, or a 6.35x43 (.257). If you go back and read The Black Rifle Volume I, one of the Army Ordnance Engineers felt that they were trying to get too much for the case size of the modified .222 Remington Special, and were pushing the pressures too high.
He also recommended looking at higher BC .224 projectiles in the 62gr to 68gr weights, using a Boat Tail in order to retain energy at distance, while bringing the pressure and mv down to a more manageable level for the case size and bolt.
The cartridge he suggested as a base for this was the .25 Remington, which was basically a .30 Remington necked down. Those of us really big into the latest generation of High Performance Intermediate Cartridges know the .30 Remington to be the parent for the 6.8x43 SPC. It would have resulted in a re-design of the AR15 at the time, which most likely would have killed the AR15 SCHV demonstrators from Armalite, which was originally chambered in .222 Remington, with a 50,000 psi SAAMI MAOP.
The AR15 actually should have undergone a re-design once it was converted to .222 Remington Special (.223 Remington), but they shoe-horned the new cartridge into it, and called it good. Only problem was, 8620 bolts meant for 50k psi don't do well with a 55,000-58,000 psi cartridge that is being pushed to the limits for mv, and bolt fatigue became an issue, with lugs shearing, and cam pin holes cracking on the edge. Here was the birth of the Carpenter 158 bolt, with HPT and MPI QC processes, since the AR15 bolt is so small.
If the Army Ordnance Corps and Armalite could have looked into the future, they would hear the whines from us asking for just a little more ogive length to play with, a little more case capacity, better BC's, a more appropriate magazine, and slightly larger receiver set. If they had known that free-float would become so popular with this design, they might have beefed up the receiver face and threads length for the barrel nut. Coulda shoulda woulda.
The 6x43 or 6.35x43 could have made an excellent DM weapon as well. Once the SAW came along in the 1970's, we would have had a great 6mm SAW case already, ready for a heavier pill with an even higher BC that would smoke the 7.62 NATO.
I still love 5.56 NATO for the ammunition capacity an individual soldier can carry, but a 6mm at the same or faster speeds would have been a killer SCHV round.
