Well, Mak, theoretically what you say makes sense but in the real world it has no meaning. I have proved that to my satisfaction many times over during 40 years of casting and loading, and not with just the .30/30 but with several other cartridges with much shorter necks, including the .300 Savage. I make no bones about liking heavy bullets in these calibers and have experimented with weights up to 220 grains, mostly cast bullets. If the effect you're referring to really takes place, the effect would be magnified by using a soft lead bullet. This argument has been batted around ad nauseum in forums and in print for many years and the consensus is always: don't worry about it. Sub-MOA accuracy with many loads in the 180-200 grain weight range speak for themselves.
When hunting with a .30/30 or .303 Savage I want every last bit of performance available to me without stepping into the realm of high pressures. That's why I favor 190's in the .30/30. I applaud Buffalo for providing a load of that weight commercially. Maybe more people will now try that weight bullet and see what they've been missing all these years.
Don't forget, the original factory load for the .303 Savage was with a 190 grain bullet. Hunters "back in the day" acknowledged that the .303 Savage had a slight edge performance-wise over it's nearly identical (in terms of case capacity) cousin the .30/30. That 190 grain bullet was why.
