[ireload2]

If you use a lighter bullet with the appropriate powder I suspect the only thing that might happen is short cycling.

 

[ireload2]

The M1 was not designed around a 190gr bullet.
Your M1 was DESIGNED for GI ammo and modified by a gun mechanic.
In terms of functioning the action with a 190 gran bullet nothing much matters except the gas port diameter and the time pressure curve of the rounds fired. If the port pressure is too high or too long it will damage the op rod. If the port pressure is too low the action will short cycle.
Why is it so difficult for you to understand how an M1 operates.
Neck sizing of the ammo buys you nothing but trouble. Learn to properly FL size your ammo and the rifle should shoot fine. But I would not shoot 190 grain bullets in it. You have little to gain by doing that. It was not designed for that.

Please please please read the posts above? It's a dedicated, triple pillared rifle designed around the 190gr projectile. Mr. McCoy had been building winning competition rifles for the US Navy shooting teams since the 60's. I suspect that he might have known what he was doing?

 

[ireload2]

It is pretty clear you know nothing about design and McCoy not much more.
Read about John C Garand's testing of the design. McCoy was no design engineer.
It is guaranteed that McCoy did not test many rifles under harsh operating conditions if he only built 3.
It is true that you cannot second guess McCoy. That you cannot even answer the question about the bullets explains a lot.

I understand perfectly. It's a good bet that you have no idea who the builder was, his history or records. The oprod and the entire rifle was built for the 190. Period. McCoy knew exactly what he was doing,. The fact that it's all explained in detail in the video he made for each individual one of these three rifles and the fact that the other two proved themselves at Perry long ago is the underscore that his rifles perform as intended. And using 190s, as specified by him.
My only question at the outset of all this was about what less expensive projectile profile I could use for that purpose besides the expensive SMK 190s.
I got my answer. The rest is all conjecture from different reloaders that means nothing to the build and intent of the rifle.
I'm in no position to second guess that master rifle builder, but I am a shooter, and a decent one at that.