I've been loading for my Socom for over 10 years, with some success... and some failures.
1) As was mentioned above, gas volume is key for the M1a action. Sticking with powders in the IMR3031-IMR4895-IMR4064 range will keep you out of trouble unless you are just hard on the furniture. I would not recommend Varget to a novice M1a reloader, it's a powder that requires some skill to load with in the M1a (that's my OPINION.) BL-C(2), TAC, W748, most powders in that burn range are good.
2) Brass is a big question, most say use military brass because it's thicker, I agree... except I would avoid using anyone's 'once-fired' brass. If it is once-fired military, it was very likely fired in a machine gun; you can beat it back into shape, but sometimes this can be more trouble than it's worth. Some champion LC (Lake City) military brass, because of the experience I have had with it, I am NOT a big fan of it... and I have pretty much sworn off it. Now that Winchester is running the LC plant, things may change... we will see. The majority of my M1a brass is from MY once-fired Prvi 145grn FMJ 'white box' ammos. Be careful about buying some older surplus... much of it is Berdan primed and not reloadable without special tools. I have some RG (Radway Green, British) surplus still left... it's my blasting ammo when I know I won't be able to recover the brass. Like the rifle itself, the brass will require some amount of prep in between loadings, mostly trim-to length and looking for cases that are separating. Military brass is heavier than typical commercial brass, which translates to less internal volume. You have to develop your loads accordingly; do not work up in commercial cases, then arbitrarily switch to military brass without reducing and working back up.
3) I like the 150's for blasting, but I'll load 168grn BTHP's for accuracy work. Lighter bullets work, too. You can shoot 175's in the M1a, I do not. My standard M80 equivalent load is (was) 43.5grn IMR4895 and any ~150grn bullet in Prvi brass. I say was... when I have worked through my 32# of IMR4895, I'll be moving to IMR3031 for the Socom, or possibly TAC, but that will be a while. H or IMR4895 is still a very good powder to start with.
4) Rifle mods. Best and easiest way to improve accuracy on a Socom is to shim the front gas lock... about a $5 mod. I also have a aftermarket (Sadlak) gas piston and guide rod, but those are not necessarily necessary. I have replaced the rear aperture with a standard. I do not intend to put optics on mine, so I've removed the rail mounted on the barrel. It was also hitting the corner of the op rod and nicking it... something to look at if you intend to keep the rail on the gun.
5) Maintenance. As was mentioned, get a good book on the M1a for maintenance. You will need appropriate drill bits to clean the gas port and gas plug. You should have a bolt disassembly tool, a ratcheting chamber brush, and a bolt roller grease thingamajig.
6) Only buy CMI (Checkmate Industries) magazines.
7) Buy lots of ammos. Once you start pulling the trigger, you won't want to stop.
