I think it'll only work when you're in one of your settlements, and can't be used everywhere. (Not sure, never tested it) Or if you're outside the settlement, fire into it. I haven't used it more than once, only 2 came and they weren't much help. Haven't used it since.
So it's probably only for settlement defense.
They work at approximately 3 squares range from closest minutemen settlement (look at the grid on a map). Range is calculated from position flare went of, so you can extend it to approximately 4 if you fire it towards settlement at a 45 degree angle.
Group of minutemen (sometimes with dog) will come running from that settlement to you. Not sure what their number is depended on, but they are at last 3 of them. They ale low level, with basic weapons and usually no armor so don't expect much from them on their own.
You can summon them only once per day.
Before they come, they will also fire flare, that's how you know if they are responding to your call or not.
If there are no enemies, they will leave after shot while.
From what I have read the Minutemen that arrives are very low level; level 1 - 3 if I am not mistaken. I never bothered using it because they would just be cannon fodder. If you are low level, I suppose they can be useful.
I left the flare gun at Sanctuary. If it is ever attacked by deathclaws again, I might decide to use it so that I can see them in action while I am sipping a Nuka Cola.
It works. Once time one of my settlements was being attacked and as I approached my settlement I fired 2-3 flares into the air and several Minute Men came to help out with the battle. One or two even died, letting you loot their equipment.
One thing it is NOT particularly useful for, is shooting down Super Mutants. One time, my wife was distracting me and I was peltering this SM as he was charging on me, and I was like "DAMMIT! I'm shooting him with flares!"
Apparently they are very "cool burning" flares.
Actually do use some thing like that to pre-heat them before starting when its really cold out.
Its pretty worthless all things considered. Artillery is at least *mildly* effective.
But half the time the MM don't show up in time to actually fight, and if they do, they're usually slaughtered within seconds.
I've only ever used it twice.
The first time nobody turned up.
The second time a Minuteman turned up but he was squashed by a Warlord in seconds.
I think i should have used it early on in the game when the opponents and my character were low level rather than later in the game where my character is called upon to assist the Minutemen from being wiped out.
I wish i could equip them better out in the field.
I sometimes drop weapons, armour and ammo at their feet in the vain hope that they will pick them up as i have seen them pick up weapons dropped by foes.
why won't MM come to College Square? I have settlements close by