M14 advantages
1. increased modularity of assembly, fewer loose parts to lose during field strip/cleaning.
2. Improved bolt design, less wear and failure, especially when the rifle is fired full automatic.
3. Full automatic capability, we all know that wasn't such a great idea now, but back then it was considered the major improvement over the Garand.
4. Detachable box magazine, increased cartridge payload, ability to clip load magazine while attached to the firearm retained.
Magazine failure or breakage does not render the weapon unservicable as is the case with the M1, the operator simply replaces the magazine with one of his carried spares.
5. 7.62X51, when standard ball cartridges are used the ballistics are nearly identical to .30/06, the difference in ability only noted past 600 yards and very few combat engagements take place past 600 yards.
The 7.62X51 weights less per cartridge than the .30/06 meaning greater number of rounds could be carried, same applies to the stripper clips, two five round loaded stripper clips weight less than one eight round en-bloc clip.
6. Improved gas system moved away from the muzzle using a seperate piston and a shorter, stiffer operating rod.
Gas piston could/can be easily replaced in the field instead of replacing the whole operating rod, the shorter, stiffer operating rod much less suseptible to bending or failure.
I love the M1 Garand but from a combat standpoint the M14 is unquestionably the better rifle.