In a movie “Killing them softly” imdb.com/title/tt1764234/?ref_=nv_sr_1 a sawed off shotgun is used during a holdup. The barrels of this side-by-side are cut so short that 2 green shotshells are actually protruding about half an inch from the muzzle. What would happen if this configuration were actually fired?
Just a guess, but I suspect that a low pressure firing would occur. The powder in a shotshell needs some barrel length to generate pressure. I had at one time an Ithaca Auto & Burglar pistol/shotgun. It had 10" barrels and developed near normal velocities with 2.5" 20ga. loads. This was an NFA registered AOW that was a '68 amnesty gun. They were originally a class III weapon but reclassified as an AOW (any other weapon in 1960) and required a C&R and a 5$ transfer tax. Was not a SBR but a smooth bore pistol. Muzzle blast was awful and very difficult to control with one hand.