If you don't have a lathe, it would be best to find some one who does and have them turn down the spacer for you. If that is not and option, you could always chuck it up in a drill press and use a file to reduce it, although this will be a slower process, you can do it yourself. Just be sure to measure the spacer first and only take off about half of the extra metal the first time, then go and shoot it to verify the amount you gained. You need to shoot a couple of four shot groups with it, 2 top barrels and 2 bottom barrels, then measure from the center to center of the pairs. Shoot as slowly and accurately as you can over a good solid rest. If you are not shooting real close pairs, stop and rest, because you are pulling the shots. You are checking the guns' progress, not trying to see how well you can shoot it offhand. It may help you if you set the bottom barrel to shoot right over the top barrel first, before you try and set the elevation spread.