The chamber is considerably larger than 30 cal. A dirty chamber is what causes the action to lock. The chamber is the part you clean with a 20 ga brush. The barrel is 30 cal. Hold a cartridge in your hand. You have the neck where the bullet is seated, that part is 30 cal. The rest is much larger than 30 cal. The entire chamber needs to be clean.
"I do not aim with my hand; I aim with my eye. He who aims with his hand has forgotten the face of his father.
I do not shoot with my hand; I shoot with my mind. He who shoots with his hand has forgotten the face of his father.
I do not kill with my gun; I kill with my heart. He who kills with his gun has forgotten the face of his father."
I just got my crate of ammo for my Mosin Nagant. I tried pushing a round down into the magazine and it won't go. It appears there is a spring loaded metal bar in the way. I do not have stripper clips which may push that aside. I then put a round in the chamber and it slid all the way in easily. The bolt won't close at all. It pushes forward but will not rotate down to the locked position. When I open the bolt, the round falls out in most cases or with a little tap.
I cleaned the crap out of it when I got it and put an ATI stock on it. Is it just not clean enough?