A single bullet cannot act as a bore obstruction in the usual sense. Read Hatcher.
Alternate theory, and I emphasize THEORY. I don't know.
Assume that the idea that incomplete ignition produces enough heat to decompose nitrocellulose into a mix of explosive gas but not enough temperature to ignite the gas (Kikkonen again, more or less) is correct. Pressure rises as slow decomposition continues and eventually the compression raises the temperature above the point of ignition (diesel effect). Ignition propagates through the gas almost instantly and BOOM!
Now allow the bullet to move forward. The volume of gas increases but there's more space for the gas to occupy. Less compression, less heat, cooler gas temperature. Perhaps the gas never gets hot enough to ignite and the bullet pops out of the barrel.
Ok, now the bullet encounters some resistance to its movement. Could be rifling, rough throat, additional friction from being copper jacketed or whatever. The bullet slows appreciably, space for the gas to expand doesn't increase as quickly, pressure, compression, and temperature rise. Possibly to the ignition point of the gas.
As far as bullet weight, remember that in the .243 WCF fiasco the powder was SLOW FOR BULLET WEIGHT. Heavy bullets have greater inertia and are slower to move forward, allowing less room for gas expansion. Pressure and temperatures rise more quickly and the nitrocellulose is quickly and completely decomposed.
Let me emphasize this is all pretty much guesswork, but seems to fit (so far).
The hard part in thinking about this is that it all happens very fast, much faster than things in our common experience. Even the slowest event is over in less than a tenth of a second. And that is much slower than the normal course of events, where the round fires properly.
