Evaluating a shooter Garand
First, check to see that it's unloaded. Pull back the bolt and leave it locked back.
When looking at Garands for shooting, not collecting, take a dummy .30-'06 cartridge with you, an M1 en-bloc clip and a cleaning rod with patch. Gently insert the bullet of the cartridge in the muzzle. A barrel with a worn muzzle will admit the bullet all the way down to the cartridge mouth. A new barrel will show about 3/8" of bullet. Put your thumb over the bolt face, get some light on your thumb and look down the bore at your thumb examining the rifling for corrosion or other problems. If you think there's just dust ior grease in the bore, use the rod, but don't let it touch the rifling. Some people claim to have Garands with bad muzzles or bores that still shoot well, but I have not seen one.
This next part be careful or you may discover what an "M1 Thumb" feels like. Test whether clips eject. Grasp the rifle stock on the under side ahead of the action. Insert the clip in the open Reciver, back end just ahead of the bolt face. Place the blade of your right hand against the handle of the Operating Rod and hold the Op Rod firmly back against its stops. Place your thumb on the Follower and the clip and gently push them down until the Bolt can be eased forward over both. Ease the Operating Rod handle and Bolt forward until they are on top of the Follower and clip, then remove your thumb from on top of the follower and clip. Once your thumb is out of the way, let the Op Rod handle slip off the blade of your hand and the bolt will slam. Tap the rear of the Op Rod handle to make sure the bolt is closed. Pulll the Op Rod handle smartly to the rear, and the clip should be ejected with a loud "Ping!"
Many stocks have been abused and rasped down, and the receivers are loose in the stock. Put your right hand around the wrist of the stock, and your left thumb against the side of the rear of the receiver and push hard: if you see the receiver move the receiver is very loose in the stock side-to-side and will shoot badly. If the rear of the receiver overhangs into space at the rear because so much wood has been rasped away, it's not going to shoot well. Sit down, put the rifle on your lap, flip the rifle over, grasp the Trigger Guard at the rear, pull it rearward and then upwards: If it merely unlatches and sits there against the Trigger Housing the wood is compressed and/or the trigger guard lugs are worn, resulting in a lack of clamping force when the Trigger Guard is closed, and the rifle will not shoot for sour owls. A new stock, with a new Trigger Guard lugs will give 1/2 to 3/8" of gap between the tip of the Trigger Guard and the Trigger Housing, and to close it you'll have to use a good deal of force to close the Trigger Guard. After doing this, pull the rear of the Trigger Guard rearwards and down to reclamp.
Pull the operating rod handle rearwards and place your hand on the gas cylinder. If it rattles even a little, it will not shoot well until the splines are peened a bit so it is tight.
When picking up a Garand, do NOT pick it up by the front handguard. Most Match conditioned Garands have the front handguard glued and sometimes screwed to the lower band, so they should not move. The attachment by screwing/gluing of handguards in Match conditioned Garands has the reputation of being somewhat fragile. You break it, you may have bought it. All other Garands, there ought to be a little play fore-and-aft in the front handguard, and those that are tight have the reputation of splitting. An old US NRA publication encouraged those offended by the rattle to cut a piece out of an old felt hat, oil or grease it, and insert it between the lower band and the front hand guard.
Before you buy it, ask if you can shoot it. Do not use modern Commercial ammo, as the powders used now are much slower and will result in gas port pressures much higher than the system was designed to operate with: Bent Op Rods and poor functioning will result. Try and find some US surplus made after about 1955 (so you can be sure it is not corrosive). If not, PMC used to make some new Commercial FMJ intended for Garands. There was some Korean surplus around a while back that was appropriate, that with the KA headstamp was corrosive, that with a PS headstamp was not. The proper powders to use are in the 3031-4895-4064 speed ranges. If handloading, full length size, have your primers a bit below flush, and use a harder primer.
Bad things that can happen when shot include poor grouping, the Operating Rod dismounting itself due to a worn tab, pierced primers, lots of different failures to function, doubling, and other problems. Doubling may just result from a failure to pull the trigger straight to the rear, or it may be worn parts such as the Hammer and/or Sear.
The rifle may well not function if not greased. Any place the working surfaces have polished through the Parkerizing needs grease, and that includes the side of the barrel where the Op Rod bears against it. Moly DiSulfide works fine. Don't put on much, as it tends to fling it all over the right lens of your glasses.