he cleaned the gun and felt the tight patch "jump" the bulge.
That'll tell you if you have a bulge in the barrel at some point, but in reality, it's hard to 'bulge' a S&W bbl.
Back in the mid-60's I was the head range officer at the Air Force Academy's range. On the day in question we were introducing new cadets to the issue S&W M15, shooting 130 gr. FMJ, military, Remington ammunition. At the end of a string of 5 shots, one shooter raised his hand with a malfunction.
On inspection, we found a round nose 130 gr FMJ bullet sticking out of the muzzle and we couldn't open the cylinder. A faint like of copper showed in the gap, so the range gunsmith got a drift and tapped the protruding bullet nose back in, and down the bore til the one that was in the cylinder gap moved enough to open the cylinder.
As it turned out, we figure the new shooter had a squib load on the first shot, but being new, didn't know it. He fired the rest of the rounds pushing the squib down the bore til it protruded from the muzzle, all five of them!
The gunsmith tapped all of the bullets back down the bore and out through the cylinder opening, checked the bore, forcing cone and cylinders but could find no damage. I loaded all six chambers with rounds and shot a group that measured less than 3" at 25 yds. The gun was fine and showed no adverse effects.
We surmised that the cylinder gap allowed enough pressure to escape that there was no buildup in the barrel enough to bulge it. Smith builds some stout guns, but I'd not recommend a steady practice of continued shooting following a squib...LOL.
In the OP's case, if indeed he had key-holing, I'd suspect under-powered, low grade ammunition, or a severely damaged muzzle crown.
Rod