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my ics m4 r.i.s. has never given me a lick of problems (stock) and upgraded the motor is being worked pretty hard on an over-achieving m120 spring. and there are literally hundreds around my area and i've not repaired one and only installed 2 upper gearbox kits so far. will they run in the 500 fps area? no. will they run in the 400 fps area. maybe, with some very careful shimming and good parts. it takes me 1 hour just to shim it with new bushings, where i can do a ver 2 in 10 minutes.
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