The mouth of the .308 case did not encounter the neck of the .270's chamber by about .050". This allowed the .308's neck to release the larger bullet while the case expanded to fill the longer chamber. This necessary expansion of the case increased the volume of the .308 'case' to the point the overall pressure was probably reduced from a normal .308 or .270 round. The bullet merely squrted out into the leade and throat as Dan noted. Also as Dan noted, the .308 bullet would draw down and squirt out the .277" barrel just fine. In this situation, the case neck was able to expand easily to allow the bullet to exit without increasing pressure, in fact due to the extra-large volume of the longer chamber, the pressure was likely reduced over normal .308 or .270. The rear of the .308 case was the proper size to seal the .270 chamber.
Ackley's experiments proved a properly opened chamber would allow what amounted to be a .35 Whelen with a 150gr. bullet to be fired in a .30/06 without increasing the pressure over a normal '06. I think the load was 45gr. IMR4895 - IIRC. By the time the powder peaked pressure - the bullet was some inches down the tube and now the correct size.
Detonations are a different matter. Had the neck of the case been restricted in opening to allow the bullet to depart easily, serious consequences might have 'ERUPTED".
Oops,. we're off topic. sorry.