About a year ago I put together a 45 caliber "slug gun" with a gain twist, 1.5 inch across the flats barrel, good lock a trigger and after much searching I was able to get a Unertl scope for a bit less than an arm or a leg! Since it was the large diameter model I had to buy taller mount blocks and did the drilling and taping of the mount holes w/o issues. A month ago I had the rifle out to 200 yards and noted the groups were 5 bullets nearly touching but about 4 inches right of center. I tried some additional scope adjustment but the tube of the scope was touching the left side of the rear adjustable mount. So I pulled the barrel off (hooked breach) and unmounted the scope and front and rear mounts. I used a pair of calipers beside the mount blocks and the ear was fine but the front was leaning a tad to the left!
I pulled the front mount block and checked the squareness of the block bottom and it was ever so slight off causing a lean and also causing the screws to bend slightly. I took a very light skim cut on the bottom and now the block is true. I remounted all and checked the scope cross hairs and now they were not plumb so I adjusted them so they are vertical and horizontal. My neighbor's garage has a very nice seam running both ways with I checked with my iPhone level and they are true. I bore sighted to 100 yards so it should be good?
So yesterday time to try the scope fix and after 4 or 5 shots I was now centered on the home made shoot-n-see and the scope tube was also in the center too which made me happy. My mistake was not checking the vertical of the blocks while I did check the bore sight and cross hair plump but now I now better. I tried moving the scope adjustment two clicks at a time both vertical and horizontal and the bullet impacts were right on the marks!
John