You can see photos of the prop gun in its case on the IMFDb . org webpage on the movie ROAD TO PERDITION. And several photos of Tom Hanks with the gun. In the case,
_ barrel, receiver, grip mount are a unit, with
_ foregrip,
_ trigger frame & rear grip, and
_ buttstock, four units.
The Thompson barrel screws into the receiver very tightly, torqued together, and is usually removed only if the barrel needs replacing. Normally you treat receiver, barrel, grip mount as a semipermanent assembly. The foregrip or forearm is attached to the grip mount with a screw. A detachable foregrip would not gain anything in length reduction.
But when you look at 1930s gangster era underground weapons, like the full-auto m1911 machine pistol with Thompson foregrip in .38 super and extended magazine, it is not a stretch to surmise that a clever gunsmith could devise a Thompson with quick detachable parts.