I tried those adjustable gas plugs in two of my Garands and they now reside in my tool box. At the time I was fighting gas pressure leaks on the gas piston of one of the guns and no setting of the plug would allow the gun to work reliably. That is not the fault of the adj. plug but in the process I determined the build of the plug was a bit crude in its adjustment. Once I fixed the gas piston sizing and went to the non-adjustable gas plug, the gun operated perfectly. So I tossed the adj. gas plug in the tool box to almost be forgotten.
As for the proper loads for the Garand, I refer you to the Hornady Reloading Manual (in the 8th and other editions) to the section dedicated to the M1 Garand loads. If any of these loads were a problem I am sure Hornady would have found it.
Powders there for 150-155 grain bullets are H335 (my choice), A2495, A2520, VIHT N-135, RL-12, VARGET, and lastly H4895. the velocity range is from 2400 fps to 2750 fps but I doubt the lower velocity loads would reliably operate the gun.
For 168 gr bullets all of the above plus W748, IMR4895, IMR4064,VIHT N-140, RL-15, VIHT N-150, and BL-C2. So we are not limited to just IMR4895. the velocity for these loads vary between 2300 fps and 2700 fps. I doubt the lower velocity loads would reliably operate the gun.
There are also loads for 178 gr bullets. If interested then look them up.
So we have many choices other than IMR4895 for our Garands. My choice is 45 gr of H335 behind a 147 gr FMJ Winchester bulk bullet. Works for me.
LDBennett