G. T. Thomas touched on this back in the late sixties. He noted that:
1) The primary danger of using long shells in short chambers is not interference between case mouth and forcing cone, but the higher pressures associated with the longer cases, and
2) His own tests with 2.75" Eley Grand Prix loads in 2.5" and 2.75" chambers showed no increased pressure in the short chamber - in fact, the short chambers recorded slightly lower pressures.
If the hulls in question are premium plastic, the mouths are generally on the order of 0.005" thick, compared to 0.025" for paper, there is considerably less material out front to cause obstruction. A plastic hull might smooth the transition from chamber to bore if it doesn't actually allow everything to open into the forcing cone before having to constrict again going into the bore.
