[Dynamo]

the majority of the time that a piston fails, it fails on the first tooth to about mid-way.
i like to think of pistons as a safety feature that protects the rest of the gear box. i rather have the piston fail than the gears, so i never use full metal teeth pistons. especially in a system that is running with a high rate of fire. at most i'll use a piston with half/half metal to plastic teeth.

the metal teeth are good for drawing the piston the last bit where the most pressure is excreted on it. if something obstructs the barrel and prevents a BB from being shot out this causes back pressure, and will slow the return of the piston. during full auto, this will cause the gears to engage the piston mid-teeth. if the piston teeth are plastic, they are sacrificed, if they are steel, you are looking at new gears.

at least thats how i see things, and generally it's what i have dealt with for over a decade of fixing guns.

with lower ROF set ups (generally outdoor FPS (380-450)), full metal rack is fine. if something has blocked the barrel, you find out on the first or second shot and can prevent damage to the gears.