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A stock CA is usually in the 310-320 fps range, so it's quite likely that gun was upgraded.
As for your gun, I'd suspect either one of a few things. The first thing I'd check is the battery. An M120 spring is pretty hard to pull. You have to have a good battery that can supply enough current. If you're using a cheap battery pack or a nunchuck style AA pack, you may have trouble turning that spring over. What type of battery is it? That info would be of great help.
Motor height could also be an issue. If the motor is too high then it'll slow down your mechbox significantly. If you can confirm the battery is good, then try lower the motor height a bit. A motor that's too high will impede the motor from moving at full speed.
If those things don't fix it, then I'd suspect it was shimmed too tightly and would need to be opened up and reshimmed.
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