The tube design doesn't do a ton with lower frequencies, so a directional capsule is employed. You'll see some manufacturers will label their pattern as "supercardiod/lobe" or similar.
An example is the Neumann KMR81i, a short gun. At lower frequencies, the polar pattern is supercardiod and as you move up the frequency spectrum, the pattern is modified by the tube design to a tighter, lobar pattern.
See:
Georg Neumann GmbH - Products/Current Microphones/KMR 81 i/Technical Data
There's a popup that lets you pick your frequency and see the polar pattern ("Interactive view of diagrams") above the specs.
In a longer gun, you've got a longer tube, and the frequency at which the tube starts really impacting the response is lower as a result, giving you that extra directionality.
see:
Georg Neumann GmbH - Products/Current Microphones/KMR 82 i/Technical Data
They've also got a great book on mic design:
Georg Neumann GmbH - Company/Publications/Basics of Microphone Technology