"Is there a danger that a round will fire before a M1A1 bolt is fully in battery" is a question expressed many times on the boards. The problem appearently did not exist during the war, or if it did, I can't find references to it or to any fixes. When GI guns went through ordnance level overhauls, the M1 bolt was most often replaced with an M1A1 bolt, and the new nomenclature was generally stamped on the side of the gun. This fix also served to slow the rate of fire by about 100rpm which was seen as desirable at the time.
I think most collectors go in the opposite direction, finding and installing a M1 bolt, for the faster cyclic rate and for a perceived added safety factor.
On chambering, the bolt initially engages only the top of the cartridge, pushing it toward the breech. The cartridge base is free to enter the firing pin indentation in the M1A1 bolt face in only the last fraction of an inch before seating fully.
I conducted a little informal experimentation along these lines. I loaded a single fired case into the chamber of my M1 with a USGI M1A1 bolt fitted. I then loaded a magazine with single rounds that were complete except that they contained no powder. I was trying as best I could to simulate a failure to extract. On each of many tries, the bulleted rounds (I made and tried several) would of course jam between the bolt face and the chambered case, but always did so with the base of the fresh "round" well away from the firing pin indentation.
Having said all of that, since I shoot left handed, I still go with the M1 bolt, as you can't be too careful when you face is fully exposed to a TSMG breech! I also think that the "bright" M1 bolt looks way cool in the gun.