Ok, so I looked it up.

David F. Butler in 'The American Shotgun' has a a brief blurb on exterior ballistics. His interior ballistics dissertation is much more extensive and highly recommended.

Butler was an 'engineer' at Winchester.

He says in reference to a chart that starts with 1336 fps at 3 feet (coil measurement), "This equates to slightly over 1350 fps at the muzzle".

So, vague and imprecise but no assertion of 100 fps loss in 3 feet.

"The price of good shotgunnery is constant practice" - Fred Kimble