I second Mr. Murphey's statement in Spades....Precious little metal to tamper with there at the front in WS-1 barrels regardless of gauge.....
As to the original poster's question, I concur with previous answers that the gun is well within the acknowledged range of constrictions as recorded in multiple references......
Years ago I was similarly perplexed by the smaller amounts of constriction in Winchester chokes , especially those stamped MOD and IMP MOD
The Winchester's use of .015 - .018 as IM for a 12 gauge gun was a far cry from the .026 - .030 used by FN and Remington during the same time frame, for the same choke designation...
In the same vein the 12 ga. Winchester designation of MOD using constriction ranging from .010-.015 fell far short of the .018 Remington listed and even farther from the frequently encountered ** FN with .020+ for MOD.
All part of the learning curve for those of us who do care to know more about about the tools we use afield
