Buckmark rear sight at it's limit, still shoots high

I just put williams fire sights on my buckmark camper. They look to be exactly the same size as the stock sights. I never sighted this gun in with the stock sights, just plinked at cans. When I went to zero the new sights in I found it shooting about 3" high at 15 yards consistently. I had the rear sight down all the way. I was shooting federal bulk pack ammo. Anything I should try differently? I gave up because I noticed that the sight base had a crack in it. Maybe this was causing the problem, although the crack was vertical, it didn't seem like I could push the base down any further or anything. It was shooting really consistently, just high. I have to call browning and get another sight base (only 300 rounds through the gun!).

Edit: Maybe the rear sight screw was coming loose, I'll check that tonight. It's probably the culprit now that I think about it. Do you guys use loctite on it?