Pwauahahahahah oh my, sorry. Looks exactly like my JG piston after putting a LiPo in it. Had to have it swapped out with the metal tooth one.

So my M4 shooting 28 bps with a 9.6V is just about in the "freaky" catagory.

I don't know why anyone uses any other piston than the deepfire all-titanium tooth.
It's bloody indestructible.
I've been running them in both my high fps/rof guns for 3 years and they show practically no wear. I've installed them in several other guns too and they've never crapped out.
Some people used to say that the bodies had cracked but I have never found that.

With high rof, you'll probably need to shave the second tooth as m10204 mentioned

One very important thing. Make sure you're using at least a 340 fps spring at over 25bps. Much slower than that and the piston won't get back fast enough.

The first thing that I see with your setup, is that you have too strong of a spring for a standard ratio gear set, which is amplified by the higher voltage battery. Either tune down the spring OR get a torque up gear set.

You're experiencing what's called "overdrive", where energy is built up in the motor when cranking a heavy spring.

Spring upgrade trouble occurs when a considerable amount of accumulated energy is forced through the motor to overcome the increased tension caused by the upgraded spring. The greater the spring tension, as well as higher the gear ratio so will the energy accumulated become greater. This accumulated energy is released right after the Sector Gear releases the Piston, and in a standard torque ratio gear set, the energy is expended and forces the gears to spin way faster than expected. This is the overdrive. The gear ends up spinning so fast, that the teeth end up slamming in to the piston tooth rack as it's coming back in to battery.

Two opposing forces this great usually doesn't end in something good, and something will have to give. In this case, it was your piston teeth that took the brunt of the force.

If you must maintain the velocity provided by that spring, your best bet is to torque up the gear set so that less strain is applied to the motor. The less strain that's applied to the motor, the less likely you'll experience overdrive, and you'll reduce your current load as well (for greater battery life.)

Consider, a real P90 shoots 900rpm, which is 15rps.

An M4 is 700-950rpm.

That all definitely makes sense Illusion. I got 50,000 rounds out of that gun before the axles sheared off of the sector and spur gears....which also damaged the original piston. I could be wrong, but aren't stock G&P gears standard ratio? If I was experiencing "overdrive" with this stock setup, should I have been able to get 50K rounds?

What happened here was a combination of a very powerful MOSFET, poor AOE, miss matched replacement parts, and what you described.