If you are going the mail order route and you want a shooter ... order the Service Grade (SG). If you have no preference for manufacture, get the HRA Service Grade. Most of the SG HRAs I have seen lately were good looking and shooting guns.
Of course your best bet is to visit the CMP South Store at Anniston, AL and pick out your Garand. If you are lucky, you may find a field grade with a good barrel. A lot of the field grades I have seen have terrible crowns and accuracy can suffer big time. Just remember, a field grade gun would NOT be issued to the troops while a service grade gun would be.
As SGF stated above, the lower the muzzel and throat gages, the better. I have purchased my fair share of garnands from CMP in all grades ... I highly recommend the HRA Service Grade. If will cost you a $100 more but well worth it IIHO!!
Hopefully they are nice. My first one should be here this week (fingers crossed). I ordered a field grade Springfield. Most all of the comments on the CMP forum lately have been very positive. This is my "booby prize" for missing out on the last of the Carbines. I'll update when I get it.
I bought a Field Grade at Camp perry in July. The muzzle measured under 2 and the throat under 4. I was lucky enough to be able to pick my own. The muzzle measurement of over 3 is dismal to say the least. If the muzzle of a rifle is worn, it won't shoot well.
The throat is measured by a throat erosion gauge. The numbers go from 0 to 10. Each number represents 1000 rounds of use. Such as a 5 is 5000 rounds fired. The service life of a Garand barrel is 8000 to 10000 rounds. I kept track of my M1 NM rifle's use and I got 9800 rounds out of a Springfield barrel before it lost target accuracy. The muzzle is a different matter. Most of the wear on a Garand barrel's muzzle is from cleaning rod friction. MY CMP service grade M1 rifle measures less than one at the muzzle. My Field grade M1 measures less than 2. It took me all day to find 3 M1 rifles with less than 2 on the Muzzle wear gauge and less that 5 on the throat erosion gauge.
Doctor Mann, the author of :"The Bullet's Flight" states a rifle can shoot well with a worn throat as long as it has a good muzzle. The last 2 inches of the rifle barrel are the most important. Our club has 2 M1 Rifles with a throat gauge of 7 and they shoot as well as rifles that have a throat measurement of 3 or 4. All the muzzles of our rifles are in VG condition because we always used rod guides on our cleaning rods when cleaning the rifles.
The chambers are cut short, so the chamber can be deepened by a hand chambering reamer. The indexing is done by an adjustable level or two machinist rulers. The front sight has to be removed from the gas cylinder and an adjustable level used on the top of the gas cylinder and the flat behind the rear sight on the receiver. It can be tricky. I was taught how to replace an M1 or M1A barrel by a Sgt. Fischer from Rock Island Arsenal. He ran the Army repair trailer at the Camp Perry Matches, in the 1980's.
If you travel to Camp Perry or Anniston's CMP Stores, they will not have Service Grade rifles on display. They reserve them for mail order only. I picked my own Service Grade rifle but that was in 2008. The CMP feels the people getting them by mail order have no opportunity to select a rifle, so they should have the better grade rifle available to them. Personally I would not want new wood on my M1 and Service Grade rifles sometimes have new wood. The change of wood negates any collector value of the M1.