You may find this interesting, from a Accurate reloading manual:
Background and basic Fundamentals of Shot Shell loading.
• The fundamental difference between a shotshell cartridge and a typical centre fire rifle cartridge is, that the efficiency of the shotshell cartridge is 100% dependant
on the round itself. By this we mean that all the “resistive forces” must be generated within the confines of the round itself. No assistance is provided by the gun.
• The reason for this is that the Maximum Peak Pressure is reached long before the base of the shot/wad assembly has left the case. (In the case of a CF rifle
cartridge, the peak pressure is achieved when the bullet is engraved, therefore the large contribution, as a result of leade/free-bore dimensions on the combustion
process. (I.e. bullet/bore interface fit, bullet hardness, bearing surface etc).
• The reason for this is that the critical engraving force which is so important to the dynamic combustion process present in a CF rifle caliber is totally absent in a
shotgun.
• Shotgun and typical straight-case handgun calibers are actually basically the same in their fundamental ratios and dynamics. A shotgun can be described as an
oversize low-pressure handgun caliber. That’s why the same powders are used in shotgun and handgun calibers.
• This means that the efficiency, regarding ignition and the subsequent increase in pressure, is totally controlled by the integral configuration and assembly of the
round itself. These constitute the main inertial mass (shot mass), the initial internal volume (wad design), the dynamic collapse (primary expansion) of the internal
volume (collapsible section of the wad), plus the displacing of the internal assembly and the unfolding of the fold/crimp (secondary/Final expansion).
• The way this COMBINATION interacts, will determine the efficiency Pressure impulse (Profile and time-base) and the Peak-pressure vs Velocity ratio (P/V). The
resistive force, presented by friction in a shotgun is negligible.
You will notice they claim the resistance of the barrel has nothing to do with pressure.