Indeed the inertia is challenging, but you might have something. Were the Marines independent enough to adopt a new cartridge and have the pull to pursue its own weaponry? Reminds me of how the M16 was first bought by the USAF for Security Police and once the nose was under the tent wall the Army was buggered. Ship the USMC spec Garand with this round to the Banana Wars and China Marines to experiment with towards lighter rifles. Or closer to home can we get the Airborne to call out a "specialist" rifle to trial? Bean counters might hate it but now you are talking a few hundred or thousand derivatives of a "standard" issue rifle and a separate logistic chain not threatening to the Quartermasters' "mountain" of 30-06. Or is that too cheeky?
Little over simplification here...the M-16 was purchased by Curt Lemay for SAC security considerations. What Curt wanted, Curt got...he was, literally, bigger than the Air Force. I doubt if any other person could have done that, 'cept maybe Rickover buying them for his nuclear subs.