OP, while it's not an M14, I have loaded my Garand down significantly from full power M2 ball specs and never managed to find the lower limit where it wouldn't function properly. For example, my standard 150gr bullet load is 48.0gr of IMR-4064, which is somewhat softer than M2 specs. I have gone as low as 41gr and the thing still functions 100% at that point. Recoil was far less than mild with that load.
To find the lower limit, just load 10 rounds with decreasing powder charges. For this test I would drop a grain for each one. Start maybe 5gr less than your standard loads and go down from there. It would take one range trip and 10 rounds to find a lower limit, not really that much and the only way to know what works in your rifle.
Mark