The average increase in pressure in Bell's tests, long shell fired in a short chamber, was indeed several hundred psi. Although there was at least one load he tested that showed an increase over 1,000 psi.

The typical takeaway is that it's safe to fire slightly longer shells in slightly shorter chambers (which the Brits have been doing for some time) IF you reduce the pressure of your load in order to offset the likely increase due to the longer hull.