If the MOSFET and battery lead are on the same post, and your motor is on the opposite post, then you still have 100% current going through the trigger contacts
You don't want your motor current going through your trigger switch, so the positive from the battery and from the motor must be on the same post, then it doesn't go through the switch. Your MOSFET lead is on the other side of your trigger contact.
Then your negative battery lead goes to the MOSFET switch, and from the MOSFET to the motor.
How this thing works is that your MOSFET is your switch that controls the power going to the motor, so 100% current goes through the MOSFET. And your trigger contacts simply tell the MOSFET switch when to open and close. The big difference is that your trigger contacts are running high voltage, almost no amperage (so no frying chunks out of the contacts), and your MOSFET is running high voltage, full amperage.
Your gun was wired in the normal fashion, but with a MOSFET lead on one side of your switch, so when you pulled the trigger, all your motor current went through your MOSFET AND your trigger contacts at the same time. So it wasn't doing anything at all other than being in the way.
Once you have it wired up properly, you should be able to hear a noticeable difference in your rate of fire, and you'll never burn out your trigger contacts ever again