Its pretty much impossible to balance in such a way that mechguns are actually useful for rangers in the new system without them being crazy strong for Hunters. Also helps define the two classes more where before Hunters could essentially be less accurate but more powerful rangers.
I've started working on a GC one now.
The tinkering never stops! 
The only way to control Mechguns is through drastic stat rebalances, basically throwing away the revamp Sega did for V3 and going back to the V2 system where common weapons are useless in Ultimate, rare weapons are an order of magnitude stronger than common weapons, both melee and ranged weapons have high ATP, and all monsters have 2000+ DFP. The ability to hit a single target nine times is simply not balanced, and the only way to mitigate it is to make the monsters strong enough that they don't die from mechgun combos.
I've started working on a GC one now.
The only way to control Mechguns is through drastic stat rebalances, basically throwing away the revamp Sega did for V3 and going back to the V2 system where common weapons are useless in Ultimate, rare weapons are an order of magnitude stronger than common weapons, both melee and ranged weapons have high ATP, and all monsters have 2000+ DFP. The ability to hit a single target nine times is simply not balanced, and the only way to mitigate it is to make the monsters strong enough that they don't die from mechgun combos.
Charge Vulcans are useless on V2, because a) 50 Hit on a weapon that gives 20 ATA to start with is nothing, so Hunters and Forces always miss, and b) sacrificial specials are multiplicative. If your base heavy mechgun damage is 400, then double damage with Charge will give you 800 damage, and enemies will fall quickly. If your base heavy mechgun damage is 200, then double damage with Charge will give you 400 damage, and enemies with the same HP will take twice as long to die, thus giving them the opportunity to fight back.
Enemy DFP is crucial to game balance, but the higher you make it, the worse androids become at soloing, and the more workarounds they need to remain playable.
Monster HP maxes at 32767, not 65535. You can't abandon enemy DFP when the HP cap is so low and when you can't boost Technique base damage to correspond with weapon damage. This isn't PSO2 where bosses have millions of HP and each attack does 1000+ damage.
A major downside of the V2 stat system is that regular weapon drops have no importance, because they're all useless. One nice thing about the V3 stat system is that you actually have a reason to check ???? weapons to see what special and percentages you got, because there's an opportunity for you to find something useful. On V2, that can't happen.
I'm not sure I understand, could you explain further?
No, it's never going to be fixed. You must use DFP to prevent players from exhausting monster HP too soon; it's the only way to handle damage in PSO.