[ILLusion]

The type of damage I'm referring to comes from full auto use, or even two consecutive shots with TOO HIGH RPM.

It's exacerbated by the fact that you have an MS100SP spring (which you admit is a soft spring - THAT is the issue... it's softness. It doesn't matter if it worked with an HK416), combined with a 11.1v battery (which performs faster than a 12v Ni-Cad/Ni-MH) AND a Systema Magnum motor which increases ROF even more.

Here's a basic description of what's happening:

Your gearset is spinning WAAAAAY too fast for the spring to keep up. Because it's a soft spring, it returns the piston to battery a bit slower, and as a result, the piston ends up slamming in to the sector gear teeth which have already begun swinging around on their next rotation. Two opposing forces moving against each other generally aren't a good thing... it's worse when it's teeth meeting in mid-engagement, because it's probably already raked a couple of teeth or more till it finally caught one), and if the sector gear DOES catch a tooth on the piston (maybe one or two teeth prematurely), it will send the piston to the back, and once it reaches the back, the piston's last tooth will release the piston... the problem is, assuming the sector gear caught the piston one or two teeth early, then that means there will be one or two teeth left in it's rotation. So guess what happens? The piston starts it's return forward, but rather than going for a full stroke, it meets the first of the two remaining teeth on the sector gear... it survives... sorta... bruised and a bit damaged... probably raked one or two teeth on the way, the piston pulls to the back again, releases as the tooth clears, and WHACK. Smacks in to the second tooth and repeat.

It could be happening with more teeth... or just one tooth. Hard to say. But your part list is the perfect recipe for this type of disaster, and your described damage sounds like the result of this type of damage. This is why there's so much damage across all your piston teeth, but only the last one or two teeth of the sector gear have damage. Check the first one or two teeth of the sector gear as well... there may be some premature wear on there as well, while all the middle teeth should look okay.

As far as removing the last tooth of the piston... that will only work if you also equally remove a tooth off your sector gear. Otherwise, FOR SURE the piston will slam in to the last remaining tooth of the sector gear, because the sector gear will then ALWAYS release the last tooth of the piston with one tooth left in the rotation.
Yes, you'll get slightly lowered velocity due to the short stroke... but this is only if you are running with a full unported cylinder.