[yuhaoyang]

Now now.... TM uses cheapo metal as well =p
it just depends on the power output of your spring, and the fit and finish of parts, both in production and assembly. ACM parts have inconsistant metal, as in the distribution of iron or carbon is not even, they also have imperfect finish, such as weird gear tooth finish, or incorrect dimensions of something, and they are assembled improperly =p
that's why if you just re-shim your mechbox (and use metal bushings+metal spring guide at 400 fps...) you can make it last much much longer.