Sometimes the cylinder gap is enough to prevent a barrel bulge. I was the senior range officer on an Air Force range in 1967, qualifying Academy Cadets with the S&W Combat Masterpiece when we had similar incident.
One of the Cadets firing on the line reported he couldn't get the cylinder to rotate out and was unable to clear the piece. On inspection we could see a faint copper glint in the cylinder/barrel gap, as well as a FMJ round nose bullet protruding from the muzzle.
The one sticking out was removed by our range gunsmith with a pair of pliers, and he used a brass drift to tap the others back down the bore. That freed up the cylinder as the one in the gap was pushed back into the cylinder throat. In all, there were 5 lodged in the gun.
The Cadet was a new shooter and had never fired a revolver before, and consequently had no knowledge of what normal recoil felt like and the noise of a 40 position range in full operation masked the dull pop of a squib.
We figured the squib was the first round fired, of the 5 loaded, the others stacking up behind it, til the 5th tied up the cylinder. Oddly enough, the barrel was not bulged and the gun functioned just fine when I test fired it. From rest, I managed six shots in less than 3" at 25 yds; a testament to S&W's in general and this particular .38 Special in particular. The .004"-.006" must have allowed the over pressure to dissipate before the barrel or cylinder gave way.
Best Regards, Rod