There was a Gun Digest article by a Danish engineer in the early 90s, on how choke works. It was awarded the editor's prize if I remember right. It stated that shot is affected by choke in a similar manner that water is affected by a constriction on a hose, making the central core of the shot fly out faster. This is an oversimplification from memory, the article had lots of diagrams showing the choke effects on shot and very long text analysis.
The "squeeze" theory seems to be supported by high speed photos of ammunition makers showing shot from choked barrels exiting the muzzle in a long string, contrasting sharply with the short fat columns exiting cylinder bores.
