Its the bridging that steel shot does when it enters the choke that does the damage. In other words, the steel does not compress to fit the choke, but stays the same diameter as it was in the more open barrel and creates an air hammer effect at the choke stricture which in turn may ring-bulge the barrel at that point...Geo

P.S.: Before plastic collars were perfected for steel shot, there was also the possibility of the harder steel scoring or scarring the softer steel the barrels may have been made of.