.I just don't know how far to pursue it or if there is something inherent in the design of the Garand.
The other dynamic...while I know what I can do with a scope at 100 yards...I just don't know how much my accuracy is affected by having 68 year old eyes with open sights at 100 yards. I can certainly imagine my eyes simply not seeing a 1-2 inch difference in the point of aim at 100 yards...
(TLDR: A decent sample of a rack grade gun can hold the 10 ring, which is roughly 3.5 MOA and the X ring is 1.5 MOA. Very few guns will hold the X ring for 20 shots slow prone unless they are match prepped. Many will not hold 95%, so don't feel bad if your doesn't. As far as your eyes, just use cheaters and you will be fine as long as you can focus on the front blade. I have won Gold medals in CMP games using dime store cheaters.)
You give a gun a good try and you call it what it is.... Can a gun that comes out of the rack and shoots poorly with good ammo want a different load? Sometimes, however...
This does happen but it was rare and generally if we fed a gun one of the pet loads and it didn't shoot well, that wasn't a good sign and I would just pass it up and reach for the next one. That is easy at the clubs with dozens of rifles to sort through, but not at home where it means trying a whole different gun. (BTW, this is why after 40 years of playing with these, I only recently bought one of my own from the CMP, but it has a brand new Criterion barrel and is completely rebuilt.)
To answer your question a little deeper, I will say that a good rack grade gun is a statistical thing, meaning that the best 1 Sigma guns can surprise you, but... that doesn't account for the other 5 Sigma worth does it?!?
In SR XTC, a good one can hold the 10 ring with a high X count at 300 yards. That means it shoots 10 rounds from prone-sling with a mandatory reload that is shot with 2 in the first clip, then 8 rounds, in under 70 seconds.
There is a brief cool off during the scoring, and then a second string is shot. This is worth mentioning since the barrel is still hot and the sure signs of a worn out gun will show the group open up during these stages. Many folks assume a gun will show wear at 600 but it is the rapid stages that open up first due to the heat during rapid strings.
So how big are the score rings that I am describing? I will show you the 300 yard targets that I am saying separate the good guns from the typical ones here, then add in the 200 yard target used in the CMP games where all the guns are in theory supposed to meet rules that do not allow match prep but will allow what I will call sorting parts and re-barreled guns.
Here are the 300 yard rings. A very good example can hold the 10 ring from prone sling with a good driver using "standard" ammo.
Here are the 200 yard rings used in CMP games. You will see "clean" scores in CMP Games when folks have a good sample and good loads.
If you translate the target to the 100 yard MOA level, just divide by 2 and you will see the X ring is roughly 1.5 MOA and the 10 ring is roughly 3.5 MOA.
Only about half the rack grade guns with good chamber and muzzle readings will hold 95% and I will estimate fewer than one third will hold clean, using HXP surplus ball ammo. The ones CMP sells with new Criterion barrels will typically shoot clean with HXP and better with hand loads, but even those are not always guaranteed to hold the 10 ring based on a few we have seen.
On your eyes... when I was young I didn't need anything to get a crisp focus on the the front blade. Now I do...
I can get by with simple dime store cheaters, but in prone they have to be full size glasses without a thick rim on top. The CMP rules do not allow lenses in the rifle, but there is a lens system called an SR-Microsight that fits inside the sight hood that allows you to focus on the front blade where it is allowed. There are also pistol shooting glasses with the cheater on the top instead of the bottom that allow you to focus on the front blade.
In summary, feed your gun a 168 MK using one of the Pet Loads. If it doesn't shoot fair you can try and adjust the loading up and down a little to see if it helps, but I haven't witnessed too many miracles with these. A good sample tends to be able to hold the 10 ring for rapid prone stages where a good driver has a chance to honestly evaluate the gun. If it will not hold for at least a slow prone test, then you call it what it is and just have fun. YMMV