My Marlin 336A Rippletop .30-30 has a chamber length of 2.153". My Savage 99 (including some spare headspace) measures 2.121". Others I have measured run all the way up from 2.060" (a commemorative M94 Musket). This leaves quite a gap for gas to get around the base of the bullet as it enters the rifling. With bullets seated on to the rifling, my 336A for instance will have between .100" and .125" of the bullet base, depending on case length, exposed to the hot gases at the end of the chamber on the final part of the bullet's passage into the barrel proper. [Note: .30-30 cases start life at about 2.025" or less]
This is analogous to seating the bullet base below the shoulder of the case, except the gas is likely to be hotter further down the chamber and the pressure higher.
I measure chamber length with a fully resized case from which half the neck has been trimmed. Inserted in the neck is a brass plunger with a flange on the forward end equal in diameter to the mouth of a fired case. The device is chambered with plunger extended, and the action closed. The end of the neck portion of the chamber pushes in the plunger. The device is removed from the rifle and chamber length (i.e. the device) measured with a vernier.
Excessive chamber length relative to case length seems to me to be a possible cause of accuracy loss, at least with the heavier hunting loads. A long time ago I tried to test the idea in the M99, blowing out some WRACo .32-40 cases to make long .30-30s. I did not prove anything, though this was with iron sights, loads that were not all that good to begin with, and case necks that by then were thinner than regular .30-30s. The modern day solution would be to size down some Starline 2.125" .38-55 cases, although the necks might still be a little thin.
What I would like to know is whether anyone out there has run tests and can offer observations on case length relative to chamber length as it affects grouping with heavier loads.
Incidentally, I see repeated reference to .32-40 cases made out of .30-30s being "too short". In reality their shortness is not much worse than what we experience with regular .30-30 ammo and chambers. Discussion of .30-30 case length could apply equally to .32-40 brass made from .30-30.
With thanks for any contributions.