My understanding is imperfect. Worse yet, I have a half-baked understanding, wherein I may actually know LESS than I think I do, or may have things backwards. Part of this is my recollection of things as they were explained to me by the friendly and long-suffering tech who would bring my horribly-abused AEGs back from the deaths caused by my abuse.
The basic way an AEG works is that a battery powers a motor. The motor turns. The gear train shifts the force of the rotation 90 degrees, and increases the torque from the motor at the expense of speed (I think, but I'm too lazy to actually look at the gear ratios). The final sector gear has teeth along 1/4 of it's circumference. These teeth engage the teeth on the underside of the piston, shoving it rearward. This rearward motion is resisted by the spring.
Once the sector gear rotates far enough to clear the piston, the spring shoves the piston back forward. The piston compresses air, and that compressed air shoves the bb out the barrel. The problem with the DSG is that instead of having 3/4 of the circumference having no teeth, it's closer to 1/4 with teeth, 1/4 clear, 1/4 with teeth, 1/4 clear. So, your piston has 1/3 of the time to slam forward than it used to.
So, to improve reliability, you:
A) Install a stiffer spring. The stiffer spring slams the piston forward faster, getting it in position to start the cycle again in time. But, the stiffer spring also compresses the same amount of air in less time, so the pressure goes up and your FPS climbs. You wear out your gear box more quickly, and may have to cut teeth off the piston or off the DSG to let it cycle properly.
B) Swiss-cheese your piston to reduce the mass, so that the spring can throw it forward faster. Again, you can end up with parts wearing out or breaking faster, since putting holes in stuff generally doesn't make it last longer (holes serve as stress concentrators; fatigue loading from mass imbalances; slamming a lighter piston forward into the end of the gear box harder, etc.)
And then you end up looking at short-stroking the piston, boring up the cylinder, and installing a heavier spring to fix your cycle times. Toss in a MOSFET system to do active braking for your motor for solid semi-auto fire and faster initial shots. Switch to higher-voltage lipo batteries with higher discharge (C) values.
The problem with chasing high ROF setups is that it is like taking a Honda Civic and turning it into a drag car. You can totally do it if you throw money at it. But you'll pay through the nose, and you'll break stuff at a fairly insane rate. Or at least I always did. Back when I played indoors regularly, I'd end up dumping around 1500 rounds or so in seven minutes on semi-auto. By the end of the game, the motor would be so hot that you could feel it through the pistol grip through gloves. I'd sit out for a game, run a different AEG for the next game, sit out one more, and the first AEG would be cool enough to swap lipo batteries in and run for another game. I'd generally break an AEG to the point of needing a rebuild every month or two, so figure a life cycle of 9 to 18 games per gun. On a really bad day, I managed to kill three guns in one visit. Realistically, between BBs and repairs it was costing me around $25 a game, plus the cost of admission.
I've heard that you can get insane ROFs with a PolarStar system. Personally, I know me. I'd end up with a separate set of identically expensive issues if I went that route.
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