What are the best performing loads for .308 out of an M1A? My rifle shoots the Sierra 165 HPBT Gameking as good as the 168g MK but I dont know how well it stacks up against other choices.

You're just going to have to burn some powder and see what is better.  My AR10 likes Hornady 168 BTHP despite most saying that the Federal GMM was the best shooting.  

You might get a little more info as far as where to start if we knew the twist rate on your barrel.

The 165gr gameking HP do well on deer but have tendency to explode at close range.  My wife shot a buck a few years ago inside of 20 yards.  The wound was devastating but did not exit.  The deer dropped in its tracks but did not die as quick as I like and needed to be finished off with another round.  When I cleaning out the deer it was a total mess but penetration was a little shallow.   I later shot a buck at about 50yard with the same load and the bullet performed perfectly.  This was a 30.06 BTW with a velocity of about 2800 fps.  I use the 165gr SP now.  

If it works well in your M1A I would stick with it.

OK, it wasn't an M1A but an HK 91.  Federal premium 165 gr Trophy bonded.  50 yards frontal on a 4 pt whitetail in eastern wa.  Entry at the chest, no exit.  When skinning we found the bullet just under the skin at the rump.  It had travelled the entire length of bucky through the vitals and looked exactly like the mushroom pictures in the ads.  Sold me on it, spensive tho.  ms

My M1A and I can do about 1-1.5" rested groups with surplus Hirtenberger 7.62NATO. Used to come in 200 round battle packs. I've only got about 900 rounds left. It is fantastic stuff! Wish I could find more! Get it if you can--and if you find some--please let me know where.

Thanks!

After looking at your post title, I have to say the best choice is either the 155 AMAX or 150 Nosler Ballistic Tips...if indeed you're looking for a SHTF zombie killer.  These are also good rounds for deer hunting.  

FMJ rounds will not "open" or frag.  BTHP will most likely not open as the hollow tip has more to do with aiding accuracy than rapid expansion.  Bonded bullets are designed to hold together and retain 100% of their weight.  On tough, thick-skinned animals that is what you want...or for shooting through glass.  On thin skinned animals you will get plenty of penetration but not much expansion.

Thanks, zombie killer is what I am looking for. Lousiana's recent problems were a real wake up call.

From tests by our very own board members, i'd second the AMAX...