> Long guns, in particular sound this way. [ ... ]
> But that's comparing long guns and short guns [ ... ]
The extra directivity of shotgun mikes occurs only at high-mid and high frequencies; most are approximately supercardioid until somewhere above 1 - 2 kHz. The interference tube creates a complex response pattern at these higher frequencies. For sound that is directly on axis, you'll get the manufacturer's printed frequency response curve, which may be just fine. But (say) 35 or 40 degrees off axis all bets are off, particularly with longer shotguns. The angle of sound incidence becomes quite critical. Shotguns still pick up sound from off-axis, and the upper midrange / high frequency character of that sound is often quite poor.
Thus you must always keep all significant sound sources strictly on axis. For dialog recording in the hands of an experienced boom operator, this can be manageable. But it really does take experience and constant babysitting--almost no one gets good results on their first few attempts. A good supercardioid is still preferable sonically, if you can get the microphone a little closer in.
Indoor music recording is a worst-case situation for shotgun microphones. The sound sources are typically wider than the main lobe of the microphone, so you get more of the "random-incidence" frequency response of the microphone rather than its (published) 0-degree response. With most shotguns, that's not pretty. It's much more "not pretty" for long shotguns than for short shotguns, and there are differences in quality among the different brands and types; not all shotguns of a given length are alike. But a good supercardioid will sound more natural than any of them, particularly if you're recording far from the sound source in a reverberant space.
Basically I'm saying, don't think of a shotgun microphone as "reaching farther"--it doesn't. "Reach" is a metaphor with limited usefulness and huge potential for giving the wrong impression in a case like this. Think instead about how (from what directions) the majority of the sound will arrive at the mike. The more off-axis sound you have, the more you need a microphone with good random incidence response. Look at the polar response diagrams and choose a microphone whose off-axis response is as similar as possible to its on-axis response at all frequencies.
--best regards