Kids nowadays. Back on the original Phantasy Star online we didn't have any fancy skill trees to specialize in a single weapon type and had to switch them all the time. And we liked it! 
So artistic license on my part.
Perhaps, I don't remember it. I only remember changing weapons to the strongest one I had among those whose playstyle I felt most comfortable with. With ranged weapons requiring little skills. I was mostly wondering since Martial Machine was apparently based on such a gameplay mechanism and Trinity started with Gun Maniac so I thought it would have a had a similar parallel. Artistic license is as good a reason as any.
Aerial Shooting: Sounds like the regular jump n' shoot. Coulda been a boost. Like Martial Machine, does it negate the attack for moving out of range/breaking LoE? Same question for any other counter that makes you move in response to an attack.
Yes.
I expressed myself poorly on that one. I meant "since Martial Machine that gives you movement with the counter do not cancel the attack that provoked the counter (unless a change in AC is above the attack roll or killing the target or similar; stuff not linked to the movement itself), were these Gun Maniac movement granting counters doing the same". Though if you mean for those counters' movement to be able to cancel the attack for moving out of range, then that answers the question all the same.
I thought that by using "fire" and "shoot" I was being explicit enough that it only applies to ranged attacks since you don't shoot or fire swords and whatnot but clearly I'm wrong. Added clause at the start of the maneuvers explanations.
Aye, hence the question. The terminology made it sound like it but it often followed things like "attack with a weapon" when the term ranged weapon is usually used for that kind of precision. You purposefully gave Martial Machine maneuvers that apply for ranged combat so it felt possible Gun Maniac would have had maneuvers that similarly applied to melee weapons as well. Thanks for that clause.
Chain Trigger: The bonus per chain is big and pretty much never ends unless you leave the stance for some reason, which means it keeps getting bigger if you fight the same enemy over multiple encounters. As per the original, it should only affect the user and not allies and should probably end as soon as you attack any other target (unless perhaps if you're using an AoE that includes a chained target). The bonus wasn't all that amazing in-game either until you used a finisher (strike) for strong bonus damage. And the chain is only from regular attacks.
True, but I wanted to make it more interesting than just moar raw damage.
Well... right now it is moar damage. So much it gets hard to beat. For you and all your allies. There is no special mechanic to have it applied since it applies against everyone forever until you change stance. I'd expect the bonus to last just one round even if it was only affecting the user, especially given how fast it can stack with itself and there being no maximum. Most of the +dmg stances give a bonus of about +PL in damage per attack and about half that to attack rolls if any. At PL 15 you hit thrice and you already reach a strong zone even if it only affect you with that one weapon.
And hitting only gets easier since it cranks up the attack bonus as well. Sure, the bonus farming is limited to a single weapon, but the bonus itself applies to every weapon, same for your allies, so they can all do a 15+weapons special fullattack with the bonus to everything.
As long as you can afford the ammo/energy.
Sure. Lots of ranged weapons don't cost all that much energy. Especially the built-in ones.
It can also receive additional boosts on the following rounds. How does it behave when that next attack is applying to an area or if it is something like, say, a single attack that attacks every enemy within range? Do you get an area of bullets moving around or in the later case pretty much one bullet for every space within range going around attacking stuff with a move/swift action?
Unless you have a way to extend the secondary boost's duration, it does not carry over. Each round is "fresh".
I know. Just saying you can apply boosts to the shot after the round it's been fired. I was mostly concerned about this applying to area weapons and multi-target "single shots".
Shift Period: Maybe there is something subtle I'm missing but how is that not just making the user invulnerable every turn forever? While attacking all enemies no less.
Energy for using the maneuver plus ammo/energy.
Added to the initial clause that the maneuvers demand weapons with ammo/energy and if you don't have enough to fire you can't use the maneuver even if you wanted the secondary effects.
Considering some weapons don't cost much energy, it easily turns into an objective of simply getting enough energy recovery each round to compensate for the use of the maneuver and the shots. There's a bunch of ways to pull that off. Just the arsenal options offer Reactor 4, %energy savings for maneuvers and extra energy battery. Many built-in ranged weapons cost only 5 EN per shot. The Boxer model offers a 2 EN shot weapon.
Or you set up a situation where you have a readied attack keyed to when the opponent tries to use Shift Period.
I don't see how this helps. It isn't something you can disrupt like a spell and even if you're hit, the maneuver is triggered to prevent harm and you're invulnerable. Assuming the target isn't invulnerable permanently anyway since it can be triggered during his own turn by having something automatically harm it each round. There are other maneuvers. Add a spammed Alert on top for good measure and ready your own action since your counter allows you to make attacks anyway.
Since mecha maneuvers are not expended and can be spammed if you've got the energy, you can also initiate it again in response to a readied attack.
Anyway. Only point I wanted to make is that this is spammable invulnerability. No cooldown or nothin'. You already made other invulnerability tricks and they aren't as easy to abuse as this one.
