To take your M12 down, follow these steps:
If you're right handed, hold with the right hand around the wrist of the stock and left hand hold on to the forearm and . . . 1. left hand press the magazine locking pin down. (front of mag tube), then use the portion of the lock pin that protrudes out the bottom and pull the bottom of the pin over to your left causing the mag tube to turn to the right side. The top tip of the locking pin will now be on the right side of the barrel instead of the left side. . . 2. with the right hand middle finger depress the action slide release lever (the button to open the action) and hold it up and with the left hand, holding the forearm, pull the action slide briskly forward. . .3. The mag tube should slide forward of the barrel/magazine band. The spring should hold the front of the action slide forward. But, with the spring missing hold the action slide forward and check to make certain the action slide bar has cleared the receiver and with the left hand twist the barrel and receiver extension assembly, from the bottom, 90 degress and disengage the assembly. The receiver extension will pull forward. Be carefull of the engagement threads as not to marr them.
Set the receiver half of the gun down and check the adjustment bushing by holding the muzzle down and look closely at the engagement thread portion of the adjustment bushing. You'll notice an adjustment sleeve lock and an adjustment sleeve lock screw. To take up wear in the assembly, the screw is removed and the lock is press rearward and the adjustment bushing is turned the next slot. You'll notice the teeth in the lock fit into the notches of the bushing. When the adjustment bushing is in the first notch, all the slots in the bushing will be to the left of the adjustment lock and screw. Looking down on it with the muzzle pointed down, the lock and screw are on the right side of the receiver extension. As the bushing adjustments are worn and used up, the next size bushing, with a tighter draw can be installed ( from size 0 to size 6) The number will be stamped on the bushing if it's been replaced. I'd assume yours is still with the original bushing.
What notch is the adjustment bushing on?