Just imagine if the lighter, trimmer .276 Garand had been adopted and gradually improved over the next 60 years, like the inherent design deficiencies of the AR have been fine tuned away (almost) over the past 40 years.
Imagine a lighter M14/21 chambered for .276 with a shorter barrel and a synthetic, collapsible stock.
Sad all that was lost due to excess inventory.
The Marines did look seriously at the Johnson semi-auto rifle and LMG, but the Garand got selected in the end. I've had two Johnsons over the years. The quick change barrel and stripper clip loading of the 10-round rotary magazine not withstanding, I found the M1 superior when it came to maintenance.
The worst part of the Johnson design is the two special captive pins that hold the rifle together. The front pin secures the front of the magazine, but the rear holds the butt stock on. Lose this pin and the rifle self-disassembles itself. Fortunately, there is a guy that specializes in replicating Johnson parts and he is making these pins.
The M1A from Springfield Armory (Geneseo, IL) is available in .243 Winchester caliber if you want to approximate the .276 Pedersen (7x51mm) round. VLTOR makes a really fine telescoping stock for the M1A that makes a very hand carbine when fitted to their SOCOM II rifle. I believe you can order this combo from Springfield and the accessory rail.