Did you happen to..
hand cycle the rifle (with no lube on it) about 50 times?
When I got mine in the 80's, that's what i did then detailed it and lube.
Ran like M1 should.
For now dismount action from stock, remove OP rod spring and follower arm,
the place the heel of action on edge of bench as you hold the muzzle, (if bolt is closed, the raise the muzzle and the OP rod and bolt should glide open about 35 degrees or greater, lower the muzzle and the OP rods / Bolt should close fully about the angle downward.
If not do the hand cycling with a dry rifle and then relube.
Also, take a .45 bore brush and epoxy it to a 8" lenght of garage door cable or outboard tiller cable, and polish the chamber.
Let us know what happen.
Ammo try and find the Hornady ammo that's marked "For the M1 Garand"
any US arsenal M2 ammo should function the rifle with nm problem, SOME Greek ammo (think any late HXP) is a little hotter than normal but a few round should be okay.