when i first met the arms 18 and mounted it to my first norinco m14s.... way back when...... i had issues with the mount not sitting perfectly straight in line with the action/barrel. It ended up being how i was mounting the arms.
Now when i mount them, i install the main side screw to the receiver, taking care to line up the mount with the recess grooves on the receiver. I then use my m14 combo tool.... the one that comes with your rifle.... and i just snug the bolt. Incidentally , the head on the side bolt on the arms is the same size as the gas plug.... so the combo tool is perfect.
after just snugging the mount, i then install the stripper clip parts and just snug that bolt. Then i bring the side bolt to final tightness, ensuring the mount is straight. then i tighten the stripper clip guide bolt and then set the forward tension pad...... done deal.